In a resealable, large plastic bag, combine garlic, rosemary, bay leaf & vinegar. Season ribs with salt & pepper & add to marinade. Allow ribs to marinate in refrigerator for about an hour.
Preheat oven to 300 F. Pour ribs & marinade into a baking dish; top with onion slices & drizzle with oil. Roast slowly for about an hour or until VERY tender. Remove ribs from oven & add pineapple pieces. Return to oven for another 5 minutes or so, ribs should be slightly golden brown. Serve with some steamed rice to which some peas have been added.