Preheat oven to 375 F (190°C). In medium saucepan, melt butter over low heat. Stir in flour, salt and pepper. Cook, stirring constantly, until mixture is smooth and bubbly.
Stir in milk all at once. Continue stirring until mixture boils and is smooth and thickened.
Separate eggs. Beat yolks well and add 1/4 cup (50 mL) of warm sauce mixture to egg yolks (picture A). Then combine yolk mixture with remaining sauce, blending thoroughly. If desired, add finely chopped filling ingredients and seasoning, stirring into the white sauce until well blended (see variations). Set sauce aside to cool slightly.
In a large bowl, beat egg whites and cream of tartar until stiff but not dry. Fold some of the egg whites into the sauce to make it lighter (picture B), then gently but thoroughly fold the sauce into the remaining egg whites.
Carefully pour into a 4 cup (1 L) souffle or casserole (picture C). Bake for 20 to 25 minutes or until done. Serve immediately.