Fry bacon pieces until browned; drain. Reserve 1 tablespoon of the drippings. Saute onions in reserved drippings until limp. Beat eggs with milk and salt. Stir in bacon, onions and 2 cups of the cheese, reserving the rest. Pour into a greased, shallow 2 quart baking dish. Bake, uncovered, at 350° F for 35 to 45 minutes, or until mixture is set and the top lightly browned. When almost done, sprinkle top with the remaining 1/2 cup cheese and return to oven until the cheese melts. Serve at once.