Mix all of the spice rub ingredients together in a large bowl. Remove the membrane from the back of the ribs (otherwise ask your butcher to do so) and coat both sides with the spice mix. Rub the spice rub into the ribs. Cover and refrigerate for about an hour to marinate.
Prepare the barbecue for indirect cooking over very low heat (95°C to 130°C- low and slow).
Lay the racks, bone side down or in a rib rack, on the cooking grills over indirect very low heat for 3 to 4 hours or until the meat has shrunk back from most of the bones by 5mm or more. Meanwhile, make the sauce.
In a saucepan, melt the butter over low heat. Once the butter has melted, add the onion and cook for a 3 minutes or until the onion has softened. Add the remaining sauce ingredients. Increase the heat, and cook for a further 10 minutes over high heat or until thick and sticky, stirring regularly.
After the ribs have been cooking for 3 to 4 hours, brush the racks with the sauce and continue to cook for a further 45 to 60 minutes, basting every 20 minutes.
The ribs are done when you lift a rack at one end with tongs, bone side up, and the rack bends so much in the middle that the meat tears. If the meat does not tear easily, continue to cook until it does.