Preheat oven to 35°F.
In a large mixing bowl, beat egg whites with water and cream of tartar just until stiff peaks form.
In a small bowl, beat egg yolks with salt until thick and lemon-colored.
Gently fold yolks into whites until thoroughly blended.
In a 1-inch omelet pan or skillet with an ovenproof handle, heat butter over medium heat until it sizzles.
Add eggs; gently smooth surface.
Cook until puffed and lightly browned on bottom, about 5 minutes.
Bake for 1 minutes or until a knife inserted near the center comes out clean.
Loosen edges with a spatula.
With a sharp knife, leaving bottom of omelet intact, cut along the diameter of omelet.
Spoon filling over half of the omelet.
Fold omelet over filling.
Garnish filled omelet with a dollop of sour cream.