How to Make It
Preheat the oven to 375°F.
In a large heavy pot, melt the butter over medium heat until the foam subsides. Whisk in the four and cook, whisking 3 minutes. Whisk in the milk and bring to a boil stirring. Boil the sauce 3 minutes, then remove from the heat and stir in 2 cups of the parmesan until melted.
In a 3-to 3 1/2-quart lasagna dish spread 1 cup of the sauce over the bottom. Place 3 lasagna noodles evenly over the sauce, leaving even space between the noodles. Top the noodle layer with an even layer of sauce, then continue to layer the lasagna in the same manner, ending with the sauce, until all the noodles and sauce are used. Sprinkle the remaining 1 cup of parmesan evenly over the 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 in places, about 30 minutes more. Let the lasagna stand 10 minutes before serving.