This is very very yummy! I made it last night and we all loved it. John even said it was, "good considering there is not much meat in it". The white sauce base mixes with the marinara sauce and makes it creamy and yummy...
1 jar of your favorite marinara sauce (24 oz. size)
1 cup 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. The sauce will continue to thicken as it stands.
Preheat the oven to 450 degrees F.
To make the lasagna: Stir the ricotta, spinach, 1 cup Parmesan, prosciutto, egg, salt, and pepper in a medium bowl to blend.
Add a tablespoon 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, then spread about 3 tablespoons 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 (not sure why she stipulates this, ours did just fine snug up against one another), atop the béchamel sauce in the dish. Repeat with the remaining noodles and ricotta mixture. Pour jar of marinara sauce over the lasagna rolls. Sprinkle the mozzarella and remaining 2 tablespoons of Parmesan over the lasagna rolls. 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.