Heat oven to 350*. 

Melt butter in saucepan. Blend in flour and seasonings.
Cook over low heat, stirring until mixture is smooth and
bubbly. Remove from heat.  Stir in milk. Heat to boiling, stirring constantly. Boil and stir 1 minute. Stir in cheese until melted. Remove from heat.

Using clean, dry beaters, beat egg whites and cream of
tartar until stiff but not dry; set aside. In a separate bowl,
beat egg yolks until very thick and lemon colored; stir
yolks into cheese mixture. Stir about 1/4 of the egg whites into cheese mixture. Gently fold mixture back into remaining egg whites.

Carefully pour into  1  1/2-quart deep casserole or souffle dish. Bake 45 to 60 minutes, or until knife inserted halfway between edge and center comes out clean. 

Serve immediately.

Makes 6 servings.