Place mushrooms and butter in a small microwave-safe bowl. Cover and microwave on high for 3-4 minutes or until mushrooms are tender, stirring twice. Add the lemon juice and seasonings.
Place salmon in an 8-in. square microwave-safe dish; top with mushroom mixture. Cover and microwave on high for 4-6 minutes or until fish flakes easily with a fork. Let stand for 5 minutes before serving.