In large mixing bowl, beat egg whites with water and cream of tarter at high speed until stiff, but not dry, just until whites no longer slip when bowl is tilted. 

 

In small mixing bowl, beat egg yolks with salt at high speed until thick and lemon-colored. Gently, but thoroughly, fold yolks into whites. In 10 inch omelet pan or skillet, with oven proof handle over medium-high heat, heat butter until just hot enough to sizzle with a drop of water. Pour in egg mixture and 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 250 degree F oven until knife inserted halfway between center and outer edge comes out clean, about 10 to 12 minutes. Loosen omelet edges with spatula.

 

To Serve Folded: With sharp knife, cut upper surface down center of omelet but DO NOT cut through to bottom of omelet. Fill, if desired. Tip skillet with pancake turner, fold omelet in half and invert onto warmed plate or platter with a quick flip of wrist. 

 

To Serve Open Faced: Invert pan over warmed plate or platter, or slide omelet from pan onto plate. Spoon or sprinkle filling, if desired, over top. Cut in half or in wedges and serve immediately. Note: to make a handle oven proof, wrap completely with aluminum foil.