Preheat oven to 400 degrees F. Grease a 2-quart shallow round (10-inch diameter) with butter; set aside. In large mixing bowl beat whites and 1/2 teaspoon salt until stiff peaks form; set aside.
Place yolks in bowl; beat lightly. Set aside. In medium saucepan cook and stir onions in butter 1 minute. Stir in flour. Add milk all at once. Cook and stir until thickened and bubbly. Stir about half the mixture into yolks. Return all to saucepan. Stir in cheddar, cayenne, and nutmeg until cheese melts. Remove from heat.
Add about 1/2 cup of the egg whites to the sauce to lighten. Gently fold whites-sauce mixture into remaining whites. Fold in corn and zucchini. Spoon into dish. Sprinkle with Parmesan. Bake uncovered, 22 to 25 minutes or until a knife inserted near center comes out clean. Let stand 10 minutes. Serve with Tomato Relish and, if desired, Parmesan shavings.
In a bowl combine tomatoes, green onions, lime juice, cilantro, jalapeno, salt, and cumin.