For sauce, melt butter in saucepan over medium-low heat. Add the flour and whisk for 3 minutes. Whisk in milk. Increase the heat to medium-high. Whisk the sauce until it comes to a simmer and is thick and smooth, about 3 min. Whisk in the salt, pepper and nutmeg (if desired).
For lasagna, preheat oven to 450 degrees. Whisk the ricotta, spinach, 1 c. Parmesan, ground beef, egg, salt and pepper in a medium bowl to blend.
Add a tablespoon or two of oil to pot of boiling water and boil the noodles till al dente. Drain.
Arrange the noodles in a single layer on a baking sheet to prevent them from sticking.
Butter a 13x9x2" baking dish. Pour the béchamel sauce over the bottom of the prepared dish. Lay out 4 lasagna noodles on a work surface, then spread a large spoonful (about 3 T.) of ricotta mixture evenly over each noodle. Starting at one end, roll each noodle like a jelly roll. Lay each roll in pan seam side down without touching. Repeat with other noodles.
Spoon 1 cup of marinara sauce over the lasagna rolls. Sprinkle with mozzarella and remaining 2 T. Parmesan cheese. Cover tightly with foil and bake until heated through and the sauce bubbles, about 20 minutes.
Uncover and bake until the cheese on top becomes golden, about 15 min. longer. Let stand for 10 minutes.
Meanwhile, heat the remaining marinara sauce to serve alongside.