Place butter in a glass 9-in. pie plate; place in a 400° oven for 2-3 minutes or until butter is melted. Remove from the oven. In a bowl, whisk milk, flour, eggs and salt until smooth; pour into hot pie plate. Bake at 400° for 25 minutes or until puffed and golden. Meanwhile, in a skillet, saute mushrooms and onion in butter until tender; remove and set aside. In the same skillet, cook sausage over medium heat until no longer pink; drain. Stir in broccoli and rice are tender. Stir in mushroom mixture and cheese. Cover and keep warm When pancake is removed from oven, immediately spoon filling into center and serve.