Make a crust of 30 small crushed saltine crackers and 1/2 cup melted butter. Saute until golden brown, 3 cups thinly sliced onions in 1/4 cup butter. Put on top of cracker crumbs in shallow dish. Sprinkle 1/2 pound grated sharp Cheddar cheese on top. This may be prepared hours ahead of time. When it comes time to put it in the oven, add 1 1/2 cups scalded milk to 3 beaten eggs, 1/2 teaspoon salt and 1/2 teaspoon pepper. Pour mixture over onion-cheese mixture in casserole. Bake at 350 degrees for 30 minutes.