Start by coating the bottom of a 2-quart soufflé dish with cooking spray and then dust it with the Parmesan cheese.
Over low heat, melt the margarine in a 3-quart saucepan, add the flour, stirring until smooth. Continue to cook for 2 minutes while stirring constantly.
Gradually add the milk, onion, salt, hot sauce, and nutmeg and then increase the heat to medium. Stir the mixture constantly until it becomes thickened and bubbly, and then remove it from the heat.
Beat the egg yolks in a small bowl at medium speed with an electric mixer until they become thick and lemon colored. Next, gradually stir 1/4 of the hot white sauce into the egg yolks to combine, and then stir that mixture back into the remaining white sauce. Add the spinach and shredded cheese and cook over medium heat until the cheese melts, roughly 1 minute.
Preheat oven to 350 degrees F and then combine 4 egg whites and cream of tartar. Beat them together until stiff, but not dry, and then gently fold them into the white sauce mixture.
To cook, spoon the mixture into the prepared dish and then bake for 50 to 60 minutes, until puffed and brown. Serve immediately.