Chop the onion finely. Heat oil in a pan over medium-high heat and fry the onion until translucent, about 3-4 minutes. Remove from heat and add vinegar, lemon juice, tomato ketchup, barbecue sauce, Worcestershire sauce, mustard, water, and salt. Stir to combine, bring to a boil over medium heat, then reduce to low heat and simmer for 30 minutes.
Preheat the oven to 250°F (121°C). Arrange the spare ribs in a baking dish and bake uncovered for 10-15 minutes.
Remove from the oven and pour the barbecue sauce over the ribs, ensuring they are well coated. Cover the baking dish tightly with aluminum foil.
Increase oven temperature to 200°C (392°F) and bake covered for 1 hour, removing the foil 15 minutes before the end of cooking to allow the sauce to caramelize. The ribs are done when they reach an internal temperature of 63°C (145°F).