Preheat oven to 350 degrees.
Beat egg whites only until soft peaks form. In a separate bowl, beat egg-yolks, mayonnaise and water. Gently fold yolk mixture into whites.

In an ovenproof skillet, melt butter and pour in egg mixture.

Cook over low heat without stirring for 10 minutes or until egg mixture has become lightly browned on bottom but still remains moist on top.

Bake in preheated 350 degrees oven for 5 minutes. Sprinkle with cheese and bake 1 to 2 minutes more, or until cheese melts.

Remove from oven. Loosen edges from side of pan and make a shallow cut through the center so that omelet will fold more easily.

Fold omelet in half and serve.

Suggested Variation: Stir chopped mushrooms and/or cooked ham into the egg mixture. Sprinkle with paprika or chives before serving.