Melt butter in electric skillet at 350 degrees. Add onion adn saute about 10 minutes. Slice unpeeled zucchini into 1/2 inch slices. Add zucchini, salt and pepper to onions, saute 6 minutes. Reduce heat to 220 degrees. Beat eggs slightly. Add milk. Pour eggs over vegetable mixture. Bake about 15 minutes in skillet. Top with cheese slices. Bake about 5 more minutes or until cheese melts. Serves 6.