Beat egg whites till frothy; add water and salt.
Beat till stiff peaks form.
Beat egg yolks till thick and lemon-colored.
Fold yolks into whites.
Melt butter in 10-inch ovenproof skillet; heat till drop of water sizzles.
Pour in omelet mixture; spread to edges of skillet, leaving sides higher.
Reduce heat; cook till puffed and bottom is golden, 8 minutes.
Bake at 325Â° till knife inserted in center comes out clean, 8 to 10 minutes.
Loosen sides of omelet.
Make shallow, off-center cut across omelet.
Tilt pan; fold smaller portion over larger portion.
Slip onto platter.