3 tablespoons grated Wisconsin Parmesan Cheese
1/4 cup butter
1/4 cup flour
1/2 teaspoon salt
1/8 teaspoon pepper
1 cup sour cream
7 egg yolks
1 cup (4 ounces) shredded Wisconsin Sharp Cheddar Cheese
1/2 cup grated Wisconsin Parmesan Cheese
1 package (10 ounces) frozen chopped spinach
7 egg whites
1/4 teaspoon cream of tartar
Generously butter a 13x9 inch baking dish. Sprinkle 3 tablespoons Parmesan over bottom of baking dish. Melt 2 tablespoons butter in saucepan over low heat. Blend in flour, salt and pepper. Cook and stir mixture until smooth and bubbly. Remove from heat. Stir in sour cream. Heat until thickened, stirring constantly. Remove from heat again. Beat egg yolks until thick and lemon-colored. Stir 1/4 cup sauce into egg yolks. Stir yolk mixture back into sauce. Add cheeses, mixing until well-blended.
Cook spinach according to package directions. Drain and squeeze out excess liquid. Stir into cheese mixture. Cover and set aside.
In a large bowl, beat egg whites until frothy. Add cream of tartar; beat until stiff but not dry. Stir about 1 cup of egg whites into cheese mixture. Gently fold mixture into remaining egg whites. Pour into prepared baking dish. Bake at 350° F for 20 to 25 minutes, or until knife inserted halfway between edge and center comes out clean. Serve immediately.