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 two inches. Bring to a boil, reduce heat and simmer, uncovered, skimming froth, 30 minutes.
Meanwhile, cook onion and garlic in oil in a 3- to 4-quart heavy saucepan over moderate heat, stirring occasionally, until onion is tender, about 15 minutes. Stir in remaining ingredients and simmer, stirring occasionally, 15 minutes.
Drain pork in a colander and pat dry, then arrange in 1 layer using tongs in a 13- by 9-inch baking dish. Pour sauce over pork to coat evenly, then cover dish tightly with foil. Bake 1 hour, then remove foil and carefully turn pork over with tongs and cook, uncovered, until very tender, about 30 minutes. Skim fat from sauce if desired.