Cook lasagna noodles to al dente per package directions.
Saute spinach
To make the white sauce:
In a large sauce pan, melt the butter over medium heat. Once melted, whisk in the flour and continue whisking until the paste begins to bubble and thicken, about 2 minutes.
Slowly whisk in the milk, until it is fully incorporated and no lumps remain. Raise heat to high and bring to a boil, then reduce to low and continue to simmer until sauce has thickened, about 3-4 minutes.
Stir in garlic, salt and pepper, then mix in the ricotta and parmesan cheeses, stirring until smooth. Remove from heat.
Assemble the Lasagna
Spread a thin layer of sauce in the bottom of a 9×13 inch baking dish. Lay a layer of lasagna noodles over the bottom.
Spread a layer of sauce on the noodles, and about half of the spinach, and top with mozzarella cheese. Repeat with another layer of noodles, sauce, the rest of the spinach, and mozzarella cheese. To finish top off with a layer of noodles, sauce, and mozzarella cheese.
Bake at 350˚F for about 40-50 minutes or until the lasagna is completely heated through and the cheese is melted. (Optional- broil for the last 5 minutes to get the cheese browned on top.)
Remove from oven and let set for about 5-10 minutes.
