Preheat your oven to 375°F (190°C).
Heat a large pot of water, adding a pinch of salt.
Cook the lasagna noodles until al dente.
Drain and set aside.
In a saucepan, melt the butter over medium heat.
Whisk in the flour to form a roux.
Gradually pour in the milk, stirring continuously until it thickens.
This is your bechamel sauce.
Spread a ladle of marinara sauce across the bottom of a 9x13-inch baking dish.
Layer with a few lasagna noodles.
Add a layer of ricotta cheese and a sprinkle of mozzarella.
Repeat layering with noodles, bechamel, and marinara sauce.
Top it off with remaining mozzarella and parmesan.
Cover the dish with foil and bake for 25 minutes.
Remove the foil, then bake another 15 minutes until the cheese is bubbly and golden.