Roast Beef Florentine with Spinach and Creamy Sauce

Roast beef Florentine is a hearty baked dish of sliced roast beef layered with spinach and a creamy, cheesy sauce, then browned until bubbling. “Florentine” commonly signals a dish served with spinach; versions vary, and this recipe turns deli-style or leftover roast beef into a satisfying oven meal without needing to roast a new joint.

Ingredients

1 tablespoon (15 ml) olive oil
1 small yellow onion, finely diced, about 1 cup (150 g)
2 garlic cloves, minced, about 2 teaspoons (6 g)
10 ounces (285 g) fresh baby spinach, or 8 ounces (225 g) frozen chopped spinach, thawed and squeezed dry
3 tablespoons (43 g) unsalted butter
3 tablespoons (24 g) all-purpose flour
2 cups (475 ml) whole milk, warmed
¼ teaspoon (1 g) ground nutmeg
½ teaspoon (3 g) fine salt, plus more to taste
¼ teaspoon (1 g) black pepper, plus more to taste
½ cup (50 g) finely grated Parmesan cheese, divided
2 pounds (900 g) cooked roast beef, thinly sliced
1 cup (115 g) shredded mozzarella cheese
1 tablespoon (15 ml) lemon juice

Instructions

Heat the oven to 375°F (190°C). Lightly oil a 9-by-13-inch (23-by-33-cm) baking dish. If using frozen spinach, thaw it and squeeze out as much water as possible; excess liquid can make the finished dish watery.
Heat the olive oil in a large skillet over medium heat. Add the onion and cook for 4–5 minutes, stirring, until soft but not browned. Stir in the garlic and cook for 30 seconds. Add fresh spinach in batches and cook for 2–3 minutes, tossing until wilted. If using thawed frozen spinach, stir it in for about 1 minute to warm through. Transfer the vegetables to a bowl and set aside.
Melt the butter in the same skillet over medium heat. Whisk in the flour and cook for 1 minute, stirring constantly; it should look smooth and pale, not toasted. Gradually whisk in the warm milk, adding it in a steady stream so lumps do not form. Bring the sauce to a gentle simmer and cook for 3–5 minutes, whisking, until it coats the back of a spoon.
Remove the skillet from the heat. Stir in the nutmeg, salt, pepper, lemon juice, and ¼ cup (25 g) Parmesan. Taste and adjust the seasoning. Fold in the spinach mixture.
Arrange half of the roast beef in the prepared dish, overlapping the slices slightly. Spoon over half of the spinach sauce, then repeat with the remaining beef and sauce. Sprinkle evenly with the mozzarella and remaining ¼ cup (25 g) Parmesan.
Bake uncovered for 20–25 minutes, until the sauce bubbles around the edges and the cheese is melted and lightly golden. The beef is already cooked, so avoid baking longer than needed or it may become dry. For leftovers or previously chilled roast beef, check the center with a food thermometer: USDA FSIS recommends reheating leftovers to 165°F (74°C).
Let the casserole stand for 5 minutes before serving so the sauce settles. If using freshly cooked whole-cut beef instead of cooked roast beef, cook that beef to a USDA FSIS safe minimum internal temperature of 145°F (63°C), measured with a food thermometer, and rest it for at least 3 minutes before slicing and assembling.

Tips

Keep the spinach dry. Spinach releases considerable moisture. Squeeze thawed spinach firmly in a clean kitchen towel, and let cooked fresh spinach drain briefly before adding it to the sauce.
Slice beef thinly across the grain. Thin slices warm quickly and are easier to serve. If using leftover roast, trim away any tough outer edges and remove large pieces of fat.
Prevent a lumpy sauce. Warm the milk and whisk it in gradually. If small lumps appear, whisk vigorously or briefly blend the sauce before mixing in the spinach.
Use care with salt. Roast beef, Parmesan, and mozzarella may already be salty. Season the sauce after adding the cheese, then adjust only if it needs it.

Serving Suggestions

Serve roast beef Florentine hot, spooning the spinach sauce over each portion. It pairs well with mashed potatoes, buttered egg noodles, rice, or crusty bread to catch the sauce. For a lighter plate, add a crisp green salad with a sharp vinaigrette or simply steamed green beans. Since the casserole is rich, fresh or lightly dressed vegetables balance the meal.

Variations

For a stronger cheese flavor, replace the mozzarella with provolone or Gruyère; both melt well, while Gruyère gives the sauce a nuttier taste. Add ½ cup (50 g) sliced mushrooms, sautéed until their liquid evaporates, with the onion. A pinch of red pepper flakes gives the sauce gentle heat. For a lighter topping, use ½ cup (50 g) breadcrumbs tossed with 1 tablespoon (15 ml) olive oil instead of the mozzarella, keeping the Parmesan in the sauce. You can also use cooked turkey or ham, though the flavor and salt level will change; season accordingly.

Storage and Reheating

Refrigerate leftovers within 2 hours of baking, in a covered shallow container or tightly covered baking dish. They keep for 3–4 days in the refrigerator. To freeze, portion the casserole into airtight freezer-safe containers and freeze for up to 2 months for best quality; thaw overnight in the refrigerator before reheating. Reheat individual portions covered in the microwave, stirring or turning halfway through, or warm a covered baking dish in a 325°F (165°C) oven. Heat leftovers to 165°F (74°C) throughout, checking the center with a food thermometer. Avoid repeated reheating, which can dry the beef.

Frequently Asked Questions

What does “Florentine” mean in this recipe?
In cooking, “Florentine” often describes a dish prepared or served with spinach, commonly alongside a creamy sauce. The term is used across many dishes, and recipes vary; here, spinach and a Parmesan cream sauce accompany roast beef.

Can I make roast beef Florentine ahead?
Yes. Assemble the casserole, cover it, and refrigerate it for up to 24 hours before baking. Let it sit at room temperature while the oven heats, but do not leave it out for more than 2 hours total. Because it starts cold, it may need additional oven time; check that the center reaches 165°F (74°C) if reheating cooked leftovers.

Can I use leftover roast beef?
Yes. Thinly sliced leftover roast beef works especially well. Keep it refrigerated until assembly, and bake only until the casserole is hot and bubbling so the meat does not become tough. When reheating the assembled leftover dish, bring the center to 165°F (74°C).

Why is my casserole watery?
The most common cause is moisture from spinach. Thoroughly squeeze thawed frozen spinach, or cook fresh spinach until wilted and let excess liquid drain. Also simmer the sauce until it thickens before layering.

Can I use frozen spinach instead of fresh?
Yes. Use 8 ounces (225 g) frozen chopped spinach, fully thawed and squeezed dry, in place of 10 ounces (285 g) fresh baby spinach. Frozen spinach has a denser texture, but it is convenient and works well in the creamy sauce.

Can I make this without flour?
For a gluten-free version, replace the all-purpose flour with 2 tablespoons (16 g) cornstarch. Whisk it first into a few tablespoons of cold milk until smooth, then stir that mixture into the remaining warm milk and simmer until thickened. The sauce will be glossier and slightly less roux-like.

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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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