Place spinach in a greased 8-in. square baking dish; set aside. Remove stems from mushrooms. Set caps aside; chop stems. Saute chopped mushrooms and onion in 2 tablespoons butter; spoon over spinach. Sprinkle with bacon. Top with mushroom caps. Melt remaining butter; brush over caps.
In a large saucepan, combine the milk, cheese, mustard, salt and pepper. Cook and stir over medium-low heat until cheese is melted; pour over the top. Bake, uncovered, at 350° for 35-40 minutes or until mushrooms are tender.