In a large bowl, toss bread cubes with butter and oil; transfer to a greased 13-in.x 9-in. baking dish. Sprinkle with cheeses, onions and salmon. In another bowl, whisk the eggs, milk, mustard, salt and pepper; pour over top. Cover and refrigerate overnight.
Remove from the refrigerator 30 minutes before baking. Cover and bake at 350° for 30 minutes. Uncover; bake 25-30 minutes longer or until a knife inserted near the center comes out clean. Let stand for 10 minutes before serving. Serve with creme fraiche and chives. Yield: 12 servings.