Melt butter in 10-inch skillet until sizzling. Add onions and bell pepper. Cook over medium high heat until crisp tender, about 5 to 7 minutes.Stack cheese slices; cut into thin strips and coarsely chop. Arrange 2 cups bread cubes in buttered 2 quart oblong glass baking dish. Top with half the beef, half the pepper mixture and half the cheese. Repeat layers ending with cheese. Whisk together the milk, flour, eggs and steak sauce. Pour evenly over ingredients in baking dish. Cover; refrigerate 8 hours or overnight. To bake, preheat oven to 375 degrees. Bake, covered with foil, 20 minutes. Uncover; continue baking for 20 to 25 minutes or until a knife inserted in the center comes out clean. Let stand 10 minutes. Cut into 6 servings. Leftovers will freeze.