Put oven rack in middle position and preheat oven to 350°F. Put ribs in a 6- to 8-quart pot and cover with water by 2 inches. Bring to a boil, reduce heat, and simmer, uncovered, skimming froth occasionally, until ribs are tender, about 1 hour. Transfer ribs to a 3-quart shallow baking dish and discard cooking water. While ribs are simmering, cook onion in oil in a 3-quart heavy saucepan over moderate heat, stirring occasionally, until softened and golden, about 15 minutes. Stir in remaining ingredients and simmer, stirring occasionally, 15 minutes. Pour sauce over ribs and cover dish tightly with foil. Bake 1 hour, then remove foil and bake 1 hour more. If sauce is not thickened and evenly covering dish lightly, continue to bake in 10- to 15-minute foil- and dish-free intervals until sauce thickens. Turn over ribs with tongs and continue baking until very tender, about 30 minutes. Skim fat from sauce, if desired.