Heat oven to 200C/180C fan/gas 6. Spray the aubergine halves generously with oil, then put them cut-side up in a large roasting tin with the garlic, coriander and cumin seeds. Season, then roast for 40 mins until the aubergine is completely tender. Set aside to cool a little.
Scoop the aubergine flesh into a bowl and discard the skins. Use a spatula to scrape the spices and garlic into the bowl. Add the chickpeas, gram flour, lemon zest and juice, roughly mash together and check the seasoning. Don’t worry if the mix is a bit soft – it will firm up in the fridge.
Shape the mixture into 20 balls and put them on a baking tray lined with baking parchment, then leave to chill in the fridge for at least 30 mins. Swirl the harissa through the yogurt and set aside. Can make ahead to this point the day before and kept covered in the fridge.
Heat oven to 180C/160C fan/gas 4. Tip the polenta onto a plate, roll the balls in it to coat, then return them to the tray and spray each one with a little oil. Roast for 20 mins until crisp, hot and golden. Serve with the harissa yogurt and lemon wedges, if you like.