Place the onion in a food processor and whizz to a fine purée. Tip into a bowl with the cumin, coriander, cinnamon, parsley, lamb and pine nuts. Season and work to a paste with your hands. Roll the paste into small walnut-sized balls; you should have about 20 in total. Chill for 15 minutes to firm up if time allows.
Preheat the oven to Gas Mark 6, 200ºC (400ºF). Toss the meatballs in the oil and place in an ovenproof dish. Bake for 15 minutes until lightly golden.
Meanwhile, place the chilli, garlic, passata and sugar in a jug. Season and mix well to combine. Pour over the meatballs and bake for another 30 minutes, turning the meatballs once until the sauce has slightly reduced and thickened.