Combine the oil and onion in a microwave safe bowl, and microwave on high for 2 minutes. Add the remainder of ingredients (sauces, sugar, vinegar, mustard and salt), and cook for a further 2 minutes longer.
Brush sauce over the spareribs, and rest for 2 hours.
Heat the oven to 135ºC.
Place ribs on rack in a lined (for easier cleaning) roasting dish curl side up, and cook in the oven for 2½ hours. Finish the ribs under the grill.