In frying pan, melt butter; add mushrooms and onion, stirring until all liquid evaporates, about 5 minutes. Add seasonings and flour; stir until blended. Gradually stir in milk and sherry. Blend in egg yolks one at a time. In a separate bowl, beat egg whites until stiff peaks form. Fold whites into mushroom mixture until just blended; fold in about 4/5 of cheese. Spoon mixture into buttered individual souffle dishes or single large souffle dish. Sprinkle remaining cheese on top. Bake small dishes for 25-30 minutes at 375 degrees. Bake large souffle at 350 degrees for 35- 40 minutes or until puffy and golden. Serve immediately.