In a large skillet, cook sausage and onion over medium heat until meat is no longer pink, 5-7 minutes; 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, until a knife inserted in the center comes out clean, 45-50 minutes., Sprinkle with mozzarella cheese. Bake until cheese is melted, 4-6 minutes. Let stand 10 minutes before cutting. Sprinkle with green onions.