Soak 4 wooden skewers in water for 10 minutes (or use metal skewers). In a bowl, mix the lamb with the onion, spices, baking powder, salt and tomato purée. Knead well until completely combined, then divide the mixture into 12 balls (about 50g each). Cut the aubergines widthways into 2cm thick slices (halve or quarter the slices if large). Brush the meatballs and aubergines with olive oil.
Preheat a grill to high (or heat up the barbecue). Thread a slice of aubergine onto a skewer followed by a meatball, repeat twice then finish with an aubergine slice, pushing to compress the meat between the aubergine. (There should be 3 meatballs on each skewer.) Repeat with the remaining 3 skewers.
Put under a hot grill (or on the barbecue) and cook for 15-20 minutes (turning every 5 minutes, if cooking on the barbecue) until the aubergine starts to feel soft and the kebabs are golden and cooked through, the juices run clear and there are no pink bits in the middle.