Line deep 8-inch pie plate or spring pan with pastry.
Heat butter in skillet, add onions and bacon; fry 3 to 4 minutes. Beat eggs in a bowl. Add warmed milk, drained onion and bacon, salt, pepper, mustard and parsley. Pour into pastry shell.
Cook in 375 degree oven for 35 to 40 minutes. If desired, garnish with extra slices or broiled bacon. Makes 6 servings.