Preheat oven to 350F.In medium saucepan, melt butter. Stir in flour and salt until well blended. Gradually add milk. Cook over medium heat, stirring constantly, until mixture thickens and boils. Stir spinach into remaining sauce.Combine ricotta cheese, Gruyére, Cheddar and 1/4 cup Parmesan cheese. Spread 1/2 cup spinach sauce in bottom of a 12 x 9-inch (3-quart) baking dish. Top with 3 uncooked lasagna noodles, half of the cheese mixture and a third of the remaining spinach sauce. Repeat layers. Top with remaining 3 noodles and remaining sauce. Sprinkle with reserved Parmesan cheese. Bake 30 to 40 minutes. Let stand 5 to 10 minutes before serving.