Melt butter in a large skillet over medium heat; add chopped onions, and sauté 10 to 15 minutes or until tender.
Place onions and bread cubes in a large bowl. Stir in milk, eggs, 1 cup cheese, and salt. Pour into a lightly greased 1 1/2-quart soufflé or baking dish. Sprinkle with remaining 1/4 cup cheese.
Bake at 350° for 25 minutes or until set.