Melt butter in a wide frying pan over medium-high heat.
Add zucchini, green pepper, onion, salt, basil, nutmeg, and pepper.
Cook, stirring often, until liquid evaporates (about 12 minutes).
Add flour and cook for 1 minute.
Beat together sour cream and egg yolks.
Stir in vegetable mixture and 2 cups of the cheese.
Spread in a well-buttered shallow 2-quart casserole.
Beat egg whites until stiff, moist peaks form.
Fold in remaining 2 cups cheese and spread over vegetable-custard mixture.
Bake in a 350° oven for about 55 minutes or until a knife inserted in center comes out clean.
Let stand for 10 minutes before serving.