In medium saucepan or skillet, stir together onion, green pepper, zucchini, tomato and seasoning. Cook, covered, over medium heat until vegetables are tender, but not brown, about 8 to 10 minutes. Keep warm.

In large mixing bowl at high speed, beat egg whites with water and cream of tartar until stiff but not dry, just until whites no longer slip when bowl is tilted. In small mixing bowl at high speed, beat egg yolks until thick and lemon-colored. Gently, but thoroughly, fold yolks into whites.

In 10-inch omelet pan or skillet with ovenproof handle⁺ over medium-high heat, heat butter until just hot enough to sizzle a drop of water. Pour in egg mixture. Gently smooth surface. Reduce heat to medium. Cook until puffed and lightly browned on bottom, about 5 minutes. (Lift omelet at edge to judge color.) Bake in preheated 350°F oven until knife inserted halfway between center and outer edge comes out clean, about 10 to 12 minutes.

Loosen omelet edges with spatula. With sharp knife, cut upper surface down center of omelet but DO NOT cut through to bottom of omelet. Arrange reserved vegetable mixture over half of omelet. Sprinkle with cheese. Tip pan. With pancake turner, fold omelet in half and invert onto warmed plate or platter with a quick flip of the wrist. Cut in half or into wedges. Serve immediately.

Note: To make handle ovenproof, wrap completely with aluminum foil.