In a large skillet, cook sausage and onion over medium heat until meat is no longer pink; drain.
Place bread cubes in a greased 13x9-in. baking dish. Layer with sausage mixture, red peppers, Brie and Parmesan cheeses and basil. In a large bowl, whisk eggs, cream, mustard, pepper and salt; pour over top. Cover and refrigerate overnight.
Remove from the refrigerator 30 minutes before baking. Preheat oven to 350°. Bake, uncovered, 45-50 minutes or until a knife inserted near the center comes out clean.
Sprinkle with mozzarella cheese. Bake 4-6 minutes or until cheese is melted. Let stand 10 minutes before cutting. Sprinkle with green onions. Yield: 12 servings.