Heat the oil and butter in the base of a tagine or a large, heavy-based saucepan over a medium heat, stir in the onion and garlic and cook for 2–3 minutes until they begin to soften and colour. Stir in the chillies, coriander and cumin seeds, and sugar and cook for 1–2 minutes, then toss in the baby aubergines, making sure they are coated in the spices.
Pour over the tomatoes, cover and cook gently for about 40 minutes until the aubergines are beautifully tender. Toss in half the mint and coriander, season to taste with salt and pepper, then re-cover and simmer for a further 5 minutes. Garnish with the remaining mint and coriander and serve hot with couscous and yogurt, if liked.