Add butter, let sizzle and browm lightly.
Tilt pan to grease sides.
With fork tines up and parallel to skillet, rapidly stir through top of uncooked egg.
As you stir uncooked egg zigzag fashion out to edges, cooked bits will come to center.
Shake pan constantly to keep egg mixture moving.
Cook 2 to 3 minutes.
If desired, spoon mushrooms cooked in butter, or other fillings across center; fold sides of omelet over, envelope style to hold in filling.