Pre-heat the oven to 180°C (fan)/200°C/gas mark 6.
Finely chop the onion, garlic and ginger and dice the aubergine.
Place the diced aubergine in an oven-proof dish, add 1 tbsp of sunflower oil and stir well. Bake in the pre-heated oven for 30 minutes until soft and golden brown, stirring halfway.
In a large saucepan (ideally one that is oven-proof to save on washing up) heat the remaining sunflower oil over a medium heat. Add the cumin seeds and fry, stirring, until just turning brown. Add the onion, stir well, then cover and reduce the heat to low. Cook for ten minutes, stirring occasionally, until the onion has softened. Add the garlic, ginger and spices, stir well and cook for a further minute.
Add the cooked aubergines, chickpeas, tinned tomatoes, garam masala and salt and pepper to taste. (It’s fine to add the aubergines a little later if they’re not quite ready yet). Bring to the boil, then cover and simmer for 15-20 minutes.
While the sauce is bubbling away peel and chop the potatoes. Place in a large pan of water, bring to the boil and cook until soft. Drain and mash along with the margarine and cumin until smooth.
Spread the mashed potato evenly over the top of the aubergine and chickpea mixture (transfer first to an oven proof dish if you need to). Scuff the top of the mashed potato with a fork and bake in the pre-heated oven for 30-40 minutes until the top is turning golden brown.