Heat ½ tbsp of the oil in a small frying pan over a medium heat and add the onion. Cook, stirring frequently, for 8-10 minutes until softened, then transfer to a plate to cool.
Meanwhile, place the cumin seeds in a small pan over a medium heat and cook until they start to make popping noises and release a fragrance. Transfer to a pestle and mortar and grind until powdery.
Once cooled, combine the onion with ¾ of the ground cumin and the remaining meatball ingredients. Season well, then scoop out heaped spoons of the mixture and shape into balls.
Combine the ingredients for the yogurt sauce and add the remaining ground cumin, then season to taste. Cover and set aside in the fridge.
Preheat the oven to 180°C/gas mark 4. Heat the remaining oil in a large frying pan and fry the meatballs in batches until browned all over, then transfer to a baking dish and cook for 10 minutes until cooked through. Serve with the yogurt sauce.