In a large nonstick skillet, saute onion and green pepper in butter until tender. Add the zucchini, tomato, oregano and pepper. Cook and stir for 5-8 minutes or until vegetables are tender and liquid is nearly evaporated. Set aside and keep warm.
In a small bowl, beat egg whites, water, cream of tartar and salt until stiff peaks form. Place egg substitute in another bowl; fold in egg white mixture. Pour into a 10-in. ovenproof skillet coated with cooking spray. Cook for 5 minutes over medium heat or until lightly browned on bottom.
Bake, uncovered, at 350° for 9-10 minutes or until a knife inserted near the centre comes out clean. Carefully run a knife around edge of pan to loosen.
With a knife, score centre of omelette. Place vegetable mixture and half of cheese on one side; fold other side over filling. Sprinkle with remaining cheese. Cut in half to serve.
Nutrients per half omelette serving: 197 calories, 9 g fat (5 g saturated fat), 21 mg cholesterol, 639 mg sodium, 10 g carbohydrate, 2 g fiber, 19 g protein. Diabetic Exchanges: 3 lean meat, 2 vegetable, 1-1/2 fat.