Have the butcher halve the spareribs horizontally, then cut into individual ribs.
Heat the olive oil in a large skillet over low heat. Sauté the onion and garlic for 5 minutes. Add the ketchup, mustard, vinegar, Worcestershire sauce, sugar, black and red pepper, and cayenne pepper, and cook for 20 to 30 minutes. Season with salt.
Pour the mixture over the ribs and marinate in the refrigerator for 3 or 4 hours or overnight. Grill over hot coals for 12 to 15 minutes.