Cut each full rack of ribs in half, so that you have 4 half racks. Sprinkle salt and pepper (more pepper than salt) and 1 tbsp cayenne pepper over meat. Wrap each piece in aluminum foil and place on a baking sheet. Bake for 2 1/2 hours.
Meanwhile, heat oil in a medium saucepan over medium heat. Cook and stir the onions in the oil for 5 minutes. Stir in remaining ingredients. Bring mixture to a boil, then reduce heat. Simmer for 1 1/4 hours, uncovered, or until sauce thickens. Remove from heat, and set aside.
Preheat an outdoor grill for high heat.
Remove ribs from the oven, and let stand 10 minutes. Remove the racks from the foil, and place on the grill. Grill the ribs for 3 to 4 minutes on each side. Brush sauce on the ribs while they're grilling, just before you serve them (adding it too early will burn it).