In 3-quart saucepan melt butter over medium heat. Cook mushrooms, 2 tablespoons onion and garlic in butter about 3 minutes, stirring frequently, until vegetables are crisp-tender. Stir in sherry, lemon juice, lemon pepper and salt. Cook over medium-high heat about 4 minutes, stirring occasionally, until almost all liquid has evaporated. Stir in whipping cream. Heat to boiling, stirring constantly. Boil over medium-high heat about 1 minute, stirring frequently, until slightly thickened.