Heat the butter in a 12-inch ovenproof nonstick skillet over medium heat. Add the mushrooms, onion and zucchini and cook until tender. Stir in the egg mixture. Reduce the heat to low. Cook for 5 minutes or until the eggs are set but still moist.
Heat the broiler. Sprinkle the cheese over the egg mixture. Broil the frittata with the top 4 inches from the heat for 2 minutes or until the top is golden brown. Sprinkle with the green onion.
Serving Suggestion: Serve with whole-grain rolls and a fresh fruit salad. For dessert serve lemon sorbet and meringue cookies.