Place butter in a glass 9-inch pie plate. Place in a 400 ºF 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, sauté mushrooms and onion in butter until tender. Remove and set aside. In the same skillet, cook sausages over medium heat until no longer pink; drain. Stir in broccoli and rice. Cook until tender. Stir in mushroom mixture and cheese. Cover and keep warm. When pancake is removed from oven, immediately spoon filling into center and serve.