Preheat oven to 160°C (140°C fan-forced/ 300°F).
For the BBQ sauce, combine all the ingredients together in a small saucepan, stirring until well combined. Cook for 3 minutes, stirring continuously. Remove from heat and allow to cool.
Place ribs in a large roasting pan with onion. In a large jug combine 1 liter water with ¼ cup of the BBQ sauce, stirring to combine. Pour over the ribs and tightly cover with foil. Bake in preheated oven for 2 ½ hours or until ribs are very tender.
Remove ribs from oven, drain off cooking liquid and remove onion. Turn the oven off. Pour the prepared sauce over the ribs, cover with foil and refrigerate for at least 2 hours.
An hour before serving remove ribs from refrigerator, so they come up to room temperature. Heat oven to 200°C (180°C fan-forced/400°F).
Place covered ribs in oven and cook for 30 minutes, turning after 15 minutes.
Serve ribs with a slaw and finger bowl.