Chop the aubergine / eggplant into 2.5 cm (1 inch) cubes.
Fill a large bowl with cold water and add a generous pinch of salt. Place the aubergine / eggplant in the water, making sure it is covered by the water (placing a plate over the top helps keep it submerged). Leave for 30 minutes.
Preheat the oven to 180˚C fan / 200˚C / 356˚C convection / 392˚F.
Line a baking tray with baking paper.
Drain the aubergine / eggplant and pat dry with paper towel.
Place the aubergine on the lined baking sheet and drizzle with 1 tablespoon oil. Bake in the preheated oven for 25-30 minutes, until tender.
Heat the other 1 tablespoon oil in a large saucepan or frying pan and add the mustard seeds and cumin seeds. Cook for 1-2 minutes until the seeds smell fragrant and start to pop, then add the crushed garlic, grated ginger, whole chillies, chopped onion and cook for 5 minutes.
Stir through the ground turmeric and chilli powder then add the tomatoes.
Simmer gently for 5 minutes.
Add the roasted aubergine / eggplant and continue to cook gently for another 5 minutes, adding 1-2 tablespoons of water if it looks too dry - it is supposed to be a dry curry, but shouldn't be sticking to the pan. If this happens add a splash or water.
Stir through the garam masala and check for seasoning, adding a little more salt if needed.
Serve with rice, naan and or chapati.
Store in the fridge or cool completely and freeze.