Barbecue ribs

INGREDIENTS

• 12 pork spare ribs
• 2 tbsp olive oil
• 1 onion
• 2 x 400g cans chopped tomatoes
• 3 garlic
• 50g/2oz light muscovado sugar
• 4 tbsp malt vinegar
• 3 tbsp Worcestershire sauce
• 1 tbsp tomato purée
• 2 tsp Chinese five spice powder
• 1 tsp ground cumin
• salt and freshly ground black pepper