Place the bread cubes in a greased 13-in. x 9-in. baking dish. In a small skillet, saute onion and mushrooms in butter; spoon over bread. Sprinkle with cheese, ham and flour. In a large bowl, whisk the eggs, milk, mustard, garlic powder and salt. Pour over ham and cheese. Cover and refrigerate overnight.
Remove from the refrigerator 30 minutes before baking. Bake, uncovered, at 350° for 60-70 minutes or until a knife inserted near the center comes out clean. Let stand for 10 minutes before serving. Yield: 12 servings.