2 tbsp unsalted butter ~ 4 tsps all-purpose flour ~ 1 1/4 cups whole milk ~ 1/4 tsp salt ~ 1/8 tsp pepper ~ pinch ground nutmeg
1 (15oz) container whole milk ricotta cheese ~ 1 (10oz) package frozen chopped spinach, thawed, squeezed dry ~ 1 cup plus 2 tbsp grated Parmesan ~ 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 tbsp olive oil ~ 12 uncooked lasagna noodles ~ 2 cups marinara sauce ~ 1 cup shredded mozzarella (about 4oz)
To make the sauce: Melt the utter 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.
Preheat oven to 400 degrees F.
Whisk the ricotta, spinach, 1 cup Parmesan, egg, salt, and pepper in a medium bowl to blend. Add 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.
Butter a 13x9x2 inch glass baking dish. Pour the béchamel sauce over the bottom of the prepared dish. ay out 4 lasagna noodles on a work surface, then spread about 3 tbsp 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 cup of marinara sauce over the lasagna rolls. Sprinkle the mozzarella and remaining 2 tbsp of Parmesan over the lasagna rolls.
Cover tightly with foil. Bake until heated through and the sauce bubble, 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.