Bring small saucepan of water to boiling. Add sauerkraut. Remove from heat; let stand 10 minutes. Drain.
Beat eggs, egg whites, water, salt and pepper in a bowl until blended.
Heat 10-inch nonstick skillet over medium heat. Add 1/2 tablespoon butter, half the onion and half the egg mixture. Cook 5 seconds, swirling pan. Lift up edges of egg mixture with spatula, letting uncooked egg run under. Cook until egg is set on top, 1 to 2 minutes.
Sprinkle half of the cheese, sauerkraut and beef down omelet center; fold over. Remove to plate. Make second omelet with remaining ingredients.