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 bechamel sauce.
2Preheat oven to 450.
3Whisk the ricotta,spinach,1 cup Parmesan,prosciutto,egg,salt and pepper in a medium bowl to blend.
4Add a tbsp 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.
5Butter a 13x9x2 inch glass baking dish. Pour the bechamel sauce over the bottom of the prepared dish. Lay out 4 lasagna noodles on a work surface,then spread about 3 tbsp of the ricotta mixture evenly over each noodle. Start at 1 end, roll each noodle like a jelly roll. Lay the lasagna rolls seam side down, with touching, atop the bechamel sauce in the dish. Repeat with remaining noodles and ricotta mixture. Spoon 1 cup of marinara sauce over the lasagna rolls. Sprinkle the mozzarella and remaining 2 tbsp of Parmesan over the lasagna rolls. Cover rightly with foil. Bake until heated through and the sauce bubbles, about 20 minutes.
6Uncover 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.