Toss 4 tablespoons melted butter with next 3 ingredients; set aside.
Sauté onions in remaining 2 tablespoons butter in a large skillet over medium-high heat 15 minutes or until golden brown. Add mushrooms, and sauté 5 minutes.
Stir in wine and next 3 ingredients; cook, stirring constantly, 5 minutes or until bubbly. Spoon mixture into a lightly greased 9-inch square baking dish; top evenly with bread mixture.
Bake at 400° for 15 minutes or until golden brown.