Preheat oven to 400°. Arrange bread pieces in a single layer on 2 baking sheets; bake about 10 minutes or until bread is toasted but not browned. Transfer to an extra-large bowl.
Meanwhile, cook sausage in a skillet over medium-high heat, stirring constantly, 10 minutes or until sausage crumbles and is no longer pink. Add sausage to bowl with toasted bread pieces using a slotted spoon. Reserve drippings in skillet.
Cook shallots and next 2 ingredients in hot drippings over medium-high heat, stirring constantly, 6 to 8 minutes or until tender. Stir in mussels and next 4 ingredients. Spoon mixture into sausage mixture, tossing until all ingredients are well blended. Add broth and eggs, stirring until well blended. Transfer mixture to a lightly greased 3-quart baking dish; top evenly with butter pieces. Cover with aluminum foil.
Bake 30 minutes; uncover and bake 10 more minutes or until golden brown.
