Sweet and Sour Sweet Onions

4 large Sweet Onions
1/4 cup cider vinegar
1/4 cup melted butter or margarine
1/4 cup boiling water
1/4 cup sugar

Slice the onions and arrange in a shallow 1-quart baking dish. Mix remaining ingredients; pour over the onions. Bake at 300° for about 50 to 60 minutes, or until onions are tender. Serves 4 to 6