Getting spinach into your diet has never been as easy (or as tasty) as making this lasagna. Slideshow: More Lasagna Recipes
Preheat the oven to 375°F.
In a large heavy pot, heat the oil over medium high heat until hot. Stir in the onion, garlic, 1 teaspoon salt and 3/4 teaspoon pepper, and cook, stirring, until golden, about 8 minutes. Stir in the flour and cook, stirring, 1 minute. Whisk in the milk and bring to a boil, whisking. Boil 5 minutes, then stir in the spinach and cook until wilted, about 3 minutes. Remove from the heat season sauce with salt and pepper to taste.
In a 3-to 3 1/2-quart lasagna dish spread 1 cup of the cheese sauce over the bottom. Make layers of noodles and sauce finishing with a layer of sauce. Dollop the ricotta over top of the lasagna.
Cover the lasagna dish with aluminum foil and bake until the filling is bubbling, about 45 minutes. Uncover the dish and continue to bake until the cheese is browned, about 30 minutes more. Let the lasagna stand 10 minutes before serving.