In a 10-in. broiler-proof skillet, saute zucchini and onion in butter until crisp-tender. In a small bowl, lightly beat the eggs, water, thyme, salt and pepper; pour over vegetables. Cook over medium heat; as eggs set, lift edges, letting uncooked portion flow underneath.
When eggs are completely set, top with tomato and cheese. Broil 4-6 in. from the heat for 1-2 minutes or until the cheese is melted and top is lightly browned. Cut in half to serve.