Prepare the ingredients. Gather the pork spareribs, barbeque sauce, onion, salt, and ground black pepper. Cut the onion into quarters.
Combine the ingredients. Place the pork spareribs in a large stockpot. Add both bottles of barbeque sauce, the quartered onion, salt, and black pepper.
Add water. Pour enough water into the stockpot to completely cover the ribs. Gently stir to distribute the sauce and seasonings.
Bring to a low boil. Place the stockpot over medium-high heat and bring the liquid to a gentle boil.
Simmer the ribs. Reduce the heat to maintain a steady simmer. Cook the ribs for approximately 40 minutes, allowing them to become tender and absorb the barbecue flavor.
Preheat the grill. Toward the end of simmering, preheat an outdoor grill to high heat. Lightly oil the grill grate to reduce sticking.
Transfer the ribs. Carefully remove the ribs from the stockpot using tongs. Reserve the barbecue cooking liquid for basting during grilling, keeping it hot.
Grill until browned. Arrange the ribs on the preheated grill. Cook for approximately 20 minutes, turning frequently to develop an evenly browned exterior.
Baste the ribs. Brush the ribs regularly with the reserved barbecue cooking liquid while grilling. Because the liquid has been in contact with raw pork, bring it to a full rolling boil before using it for basting.
Check for doneness. Continue grilling until the ribs are nicely browned and tender. Pork must reach a minimum internal temperature of 145°F (63°C), followed by a 3-minute rest, although ribs often become more tender at higher temperatures.