Preheat oven to 350° F.
Mix biscuit mix, mustard in saucepan.
Add small amount of milk to make paste, then rest of milk gradually.
Bring to boil; boil 1 minute, stirring constantly. Stir in cheese; remove from heat.
Stir into slightly beaten egg yolks. Beat egg whites and cream of tartar until stiff enough to hold soft peaks.
Fold into cheese mixture.
Pour mixture into an ungreased 1 1/2-quart baking dish and set the dish in pan of water.
Bake for 50-60 minutes at 350° F or until silver knife inserted near center comes out clean.
