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 Mixing 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 Nonstick Cooking Spray. Cook Over Medium Heat For 5 Minutes Or Until Bottom Is Lightly Browned. Bake At 350 Degrees F For 9-10 Minutes Or Until A Knife Inserted Near The Center Comes Out Clean. Spoon Vegetable Mixture Over One Side; Sprinkle With Half Of The Cheese. To Fold, Score Middle Of Omelet With A Sharp Knife; Fold Omelet Over Filling. Transfer To A Warm Platter. Sprinkle With Remaining Cheese. Cut In Half To Serve.