Preheat oven to 400°F. Grease a 2-quart shallow round with nonstick cooking spray, and set aside. In a large mixing bowl beat whites and salt until stiff peaks form; set aside.
Place yolks in small bowl, and lightly beat. Set aside. Melt butter in saucepan over medium heat, and add onions and yellow pepper cooking 1 minute. Stir in flour. Add milk all at once. Cook and stir until thickened and bubbly. Stir about half the milk mixture into yolks. Return all to saucepan and remove saucepan from heat. Stir in cheddar and cayenne until cheese melts.
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 the prepared dish. Sprinkle with Parmesan. Bake uncovered, 20-25 minutes or until a knife inserted near center comes out clean. Let stand 10 minutes.