Preheat oven to 350 degrees. Butter and dust a 1-1/2 quart souffle dish with grated Parmesan or breadcrumbs. Separate the eggs. Put egg whites in mixer bowl with cream of tartar. To make a white sauce, in a medium saucepan over medium high heat, melt the butter or margarine until it bubbles. Stir in the flour and cook, stirring constantly until smooth and bubbly. Stir in milk all at once, than cook and stir until mixture boils and is smooth and thickened. To flavor the white sauce, stir in the cheese until melted. Stir in the spinach. Set aside. Beat the egg whites only until they cling to the side of the mixing bowl when you roll and tilt it. Add the yolks to the sauce, and stir. To fold the sauce into the whites, first mix about a quarter of the whites right into the sauce in the pan, then empty the sauce into the whites in the mixing bowl. Gingerly but thoroughly fold the sauce into the whites. Then pour that mixture into the prepared souffle dish. Bake for about 30 to 40 minutes. Don't open oven door for at least 30 minutes. Serve immediately.