Gently fry the chopped onions in four tablespoons of oil, add the spices and stir well. Once the onions have softened, set the pan to one side.
Put some oil on a piece of kitchen paper and rub the aubergines all over, then prick them about three times with a fork, place on a baking tray and put in the oven on a high heat for about 30 minutes Check them after about 10 minutes and turn them over until they're slightly charred on the outside.
Take the aubergines from the oven and cut them down one side and take the tops off. Open them up and carefully scoop the flesh out and place in a bowl. Mash the flesh with a fork.
Now put the spice and onion mixture back on the hob and heat gently, add the mashed aubergine and stir well.
Wash the spinach, break it up into small pieces and add to the pan. Stir well then add your chopped tomatoes.
Leave for about 10 minutes on a low heat then add your crushed garlic and coriander, stir well and gently heat for another 15 minutes.
If possible leave the curry for a couple of hours to stand and reheat just before serving.