Sauce:

2 T. unsalted butter

4 tsp. all-purpose flour

1 1/4 C. whole milk

1/4 tsp. salt

1/8 tsp. ground black pepper

Pinch ground nutmeg

 

Lasagna:

1 (15-ounce) container whole milk ricotta cheese

1 (10-ounce) package frozen chopped spinach, thawed, squeezed dry

1 C. plus 2 T. grated Parmesan

3 ounces thinly sliced prosciutto, chopped

1 large egg, beaten to blend

3/4 tsp. salt, plus more for salting water

1/2 tsp. freshly ground black pepper

1 to 2 T. olive oil

12 uncooked lasagna noodles

2 C. marinara sauce

1 C. shredded mozzarella (about 4 ounces)

 

To make the sauce: Melt the butter in a heavy medium saucepan over medium-low heat. Add the flour and whisk for 3 minutes. Whisk in the milk. Increase the heat to medium-high. Whisk the sauce until it comes to a simmer and is thick and smooth, about 3 minutes. Whisk the salt, pepper, and nutmeg into the béchamel sauce. Set aside. Preheat the oven to 450 degrees F. Mix the ricotta, spinach, 1 C. Parmesan, prosciutto, egg, salt, and pepper in a medium bowl. Add a T. or 2 of oil to a large pot of boiling salted water. Boil the noodles until just tender but still firm to bite. Drain. Arrange the noodles in a single layer on a baking sheet to prevent them from sticking. Butter a 13-by-9-by-2-inch glass baking dish. Pour the béchamel sauce over the bottom of the prepared dish. Lay out 4 lasagna noodles on a work surface, and then spread about 3 T. of ricotta mixture evenly over each noodle. Starting at 1 end, roll each noodle like a jelly roll. Lay the lasagna rolls seam side down, without touching, atop the béchamel sauce in the dish. Repeat with the remaining noodles and ricotta mixture. Spoon 1 C. marinara over rolls and sprinkle with remaining cheese. Cover tightly with foil. Bake until heated through and the sauce bubbles, about 20 minutes. Uncover and bake until the cheese on top becomes golden, about 15 minutes longer. Let stand for 10 minutes. Meanwhile, heat the remaining marinara sauce in a heavy small saucepan over medium heat until hot, and serve alongside.