Melt Butter In A Large Nonstick Skillet Over Medium-High Heat. Stir In Mushrooms, And Cook Until Tender, About 5 Minutes. Using A Slotted Spoon, Transfer Mushrooms To Plate. Remove Skillet From Heat. In A Small Bowl, Beat Egg Yolks And Milk To Blend. In A Separate Bowl, Beat Egg Whites With A Dry Beater Until Soft Peaks Form. Fold Yolk Mixture Into Whites. Return Skillet To Medium-High Heat. When Butter Sizzles, Pour Egg Mixture Into Skillet. Sprinkle Mushrooms And Crab Meat Evenly Over The Top. Cook Until Omelet Begins To Set, About 3 Minutes. Sprinkle With Swiss Cheese. Using A Large Spatula, Fold Omelet Over, Forming Semicircle. Cover Skillet; Cook Until Omelet Puffs And Is Cooked Through, About 3 Minutes. Slide Omelet Onto Large Plate.