Heat oven to 200C/180C fan/gas 6. Toss the aubergines in a roasting tin with 2 tbsp olive oil, season well and spread out. Roast for 20 mins or until dark golden and soft.
Heat the remaining oil in an ovenproof pan or flameproof casserole dish and cook the onions over a medium heat for 5-6 mins until softening. Stir in the garlic and spices, for a few mins until the spices release their aromas.
Tip in the tomatoes, coconut milk and roasted aubergines, and bring to a gentle simmer. Simmer for 20-25 mins, removing the lid for the final 5 mins to thicken the sauce. Add a little seasoning if you like, and a pinch of sugar if it needs it. Stir through most of the coriander. Serve over rice or with chapatis, scattering with the remaining coriander.