In medium saucepan or skillet, stir together onion, green pepper, zucchini, tomato and Italian 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.