Clear Japanese Mushroom and Onion Soup

Clear Japanese mushroom and onion soup is a light, savory broth made with dashi, tender onion, and sliced mushrooms, then finished with scallions. It is a Japanese-style take on clear soup: simple enough for a weeknight, yet fragrant and satisfying as a starter, with plenty of room for regional and household variations.

Ingredients

6 cups (1.4 L) water
1 teaspoon (4 g) instant dashi granules, or the amount specified on the package for 6 cups of broth
1 medium yellow onion (about 5 oz / 150 g), peeled and thinly sliced
5 oz (140 g) fresh shiitake mushrooms, stems removed and caps thinly sliced
1 tablespoon (15 ml) regular or low-sodium Japanese soy sauce
2 teaspoons (10 ml) mirin
¼ teaspoon (1.5 g) fine salt, plus more to taste
2 scallions (about 30 g), thinly sliced
1 teaspoon (5 ml) toasted sesame oil, optional, for serving

Instructions

Prepare the vegetables. Peel the onion and slice it into thin half-moons so the pieces soften evenly and are easy to eat with a spoon. Wipe the shiitake caps clean with a damp paper towel, remove the tough stems, and slice the caps. Rinse and thinly slice the scallions; keep them aside for the finish.
Start the dashi broth. Pour the water into a medium saucepan and stir in the dashi granules. Add the sliced onion. Set the pan over medium heat and bring it to a gentle simmer, which should take about 3 minutes. Avoid a vigorous boil; a quiet simmer keeps the broth clear and helps the onion soften without breaking apart.
Simmer the onion. Reduce the heat as needed to maintain a gentle simmer and cook the onion for 9 minutes. Stir once or twice, lifting the onion from the bottom rather than beating it around the pan. The slices should turn translucent and become tender while still holding their shape.
Cook the mushrooms. Add the sliced shiitake and simmer for 5 minutes. The mushrooms should become tender and release a subtle earthy aroma into the broth. If foam gathers on the surface, skim it off with a spoon; this small step makes the finished soup look cleaner without removing its flavor.
Season and finish. Stir in the soy sauce, mirin, and ¼ teaspoon salt. Simmer for 3 minutes, then taste a spoonful. Add salt a small pinch at a time if it needs more; if it tastes too salty, add a little hot water. Take the pan off the heat and stir in half of the scallions.
Serve hot. Ladle the soup into warm bowls, making sure each serving gets some onion and mushrooms. Scatter the remaining scallions over the top. Add a few drops of toasted sesame oil to each bowl if you like, and serve promptly while the broth is hot and clear.

Tips

Check your dashi label and ratio. Instant dashi granules differ in strength and saltiness, so follow the package amount if it calls for more or less than 1 teaspoon per 6 cups (1.4 L) water. Some granules contain bonito, a dried fish ingredient; check the label if you need a vegetarian broth. Dashi is the savory Japanese stock that gives this soup its characteristic depth, so plain water alone will taste noticeably flatter.
Keep the simmer gentle. A rolling boil can make the broth look cloudy and can cause the onion to break into soft strands. Once the water starts simmering, turn the heat down enough to keep only small, steady bubbles around the edge of the pan.
Slice evenly and season at the end. Thin, similar-sized onion and mushroom slices cook at a similar pace. Dashi and soy sauces vary in saltiness, so taste after adding the measured soy sauce before adjusting the salt. Adding extra salt early can make it harder to balance the finished broth.
Choose a practical dashi substitute if needed. For a vegetarian version, use kombu-and-shiitake dashi or a clearly labeled vegetarian dashi powder. A mild vegetable broth can work in a pinch, but it makes the flavor less distinctly Japanese and may need less added salt. Avoid strongly flavored broths that can overpower the mushrooms.

Serving Suggestions

Serve this soup in small bowls as a first course alongside a Japanese-inspired meal, or pour a larger portion into a bowl for a light lunch. It pairs well with steamed rice, grilled salmon, tofu, vegetable tempura, or simply prepared chicken. If serving it with a salty main dish, keep the soup seasoning restrained so the meal does not become too salty overall.

For a more filling bowl, add a small portion of cooked soba or udon just before serving. Cook noodles separately and place them in the bowls before ladling over the soup; cooking them directly in the pot can release starch and make the broth less clear. A little extra scallion adds freshness, while a few drops of sesame oil provide a toasted aroma. Keep additions modest so the dashi, onion, and mushrooms remain the focus.

Variations

Use different mushrooms: Replace the shiitake with 5 oz (140 g) cremini or button mushrooms for an easy-to-find option. Their flavor is milder and less earthy, so the soup will be gentler. Enoki mushrooms also work: trim away the root end, separate the strands, and simmer them for only 2 to 3 minutes so they stay delicate. You can combine two kinds, keeping the total amount around 5 oz (140 g).

Make it vegetarian: Choose vegetarian dashi granules or prepare kombu-and-shiitake dashi, and confirm that the soy sauce and mirin suit your needs. Kombu is dried kelp; it gives the broth a clean, savory character without the bonito used in many standard dashi powders. If using a prepared vegetable broth instead, start with less salt because packaged broth can already be well seasoned.

Add tofu or greens: Stir in 4 oz (115 g) soft or medium-firm tofu, cut into small cubes, during the last 3 minutes of cooking. For spinach or thinly sliced napa cabbage, add a small handful during the last 1 to 2 minutes, just until wilted. These additions make the soup more substantial and slightly change its appearance, but they do not require a different base method.

Adjust the seasoning style: For a delicate, pale broth, use low-sodium soy sauce and begin with only half the measured salt. For a deeper color and more pronounced soy flavor, use regular soy sauce, but taste before adding any extra salt. Mirin adds a gentle sweetness; if you do not have it, omit it rather than substituting a large amount of sugar, which can make the soup taste out of balance.

Storage and Reheating

Cool the soup and refrigerate it within 2 hours of cooking, in a covered container. It keeps for 3 to 4 days in the refrigerator. The mushrooms and onions will soften further as they sit, and the broth may taste a little stronger the next day; stir it before reheating and adjust with a splash of water if needed.

Reheat only the portion you plan to serve in a saucepan over medium heat, stirring gently until steaming throughout. For food safety, USDA FSIS advises reheating leftovers to 165°F (74°C); check the temperature with a food thermometer. Avoid boiling it hard, which can make the vegetables very soft and disturb the clear appearance. You can also reheat a serving in the microwave in a covered, microwave-safe bowl, stirring halfway through and checking the temperature in more than one spot.

Freezing is possible, though the mushrooms and onions may become softer after thawing. For best texture, freeze the broth and vegetables together in a freezer-safe container, leaving a little room for expansion, and use within about 2 months for good quality. Thaw overnight in the refrigerator and reheat as described above. Do not leave the soup at room temperature for extended storage.

Frequently Asked Questions

Is this the same as Japanese clear soup served at a restaurant?
It is a home-style Japanese-inspired clear soup built on dashi, soy sauce, and tender vegetables. Restaurant clear soups vary, and some use different aromatics or a broth made from scratch. This version keeps the preparation straightforward while retaining a clear broth and familiar Japanese seasonings.

Can I use regular mushrooms instead of shiitake?
Yes. Cremini or white button mushrooms are practical substitutes, and the same amount and cooking time work well when they are thinly sliced. The finished soup will have a milder mushroom flavor. If you use enoki, add it near the end because its fine strands cook quickly.

Why did my broth turn cloudy?
A hard boil, broken-up vegetables, or starchy noodles cooked directly in the soup can cloud the broth. Keep the pan at a gentle simmer, stir carefully, and cook any noodles separately. A little cloudiness does not make the soup unsafe or spoil its flavor; it mainly changes the clear appearance.

Can I make the soup ahead?
Yes. Cook it up to 3 to 4 days ahead and refrigerate it promptly in a covered container. For the freshest color and texture, keep the scallions separate and add them when serving. Reheat gently, bring leftovers to 165°F (74°C), and avoid repeatedly warming and cooling the full pot.

What can I use instead of mirin?
You can leave it out for a less rounded, slightly less sweet broth. If you have cooking sake, use 2 teaspoons (10 ml) and add a small pinch of sugar only if you want to replace the sweetness; cooking sake is not identical to mirin. Taste before adding salt, since different substitutes can affect the balance.

Is dashi always made with fish?
No. Many traditional dashi preparations use kombu, bonito flakes, or both, and there are also vegetarian versions made with kombu, dried shiitake, or other ingredients. Instant granules vary, so check the ingredient list when fish-free soup is important. Vegetarian dashi keeps the soup savory, though its flavor differs slightly from bonito-based stock.

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Safety notes: safe minimum internal temperatures from USDA FSIS; raw and undercooked fish guidance from the U.S. FDA. Food safety notes

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