In a heavy skillet, melt butter. Add squash, zucchini and onion; cook until crisp tender. Pour squash into a casserole dish; set aside to cool.
To the slightly cooled squash, add Cheddar cheese, salt and pepper; stir.
Mix together milk, egg yolk and flour; whisk until well blended. Pour milk mixture over squash; stir. Sprinkle with additional Cheddar cheese if desired.
Bake at 375 degrees about 45 minutes until hot and bubbly throughout.