Place the aubergine slices into a colander set over a bowl. Sprinkle with salt and leave for 30 minutes to draw out the moisture from the aubergine.
Meanwhile, to a large pan over a high heat add the groundnut oil with the onions, garlic and ginger. Cover with a lid, lower the heat and cook for 20 minutes or so, until the onions have softened.
Crush the cardamom pods, extract the black seeds and lightly crush them in a mortar with a pestle. Add them to the same pan, along with the chilli flakes, turmeric and cumin seeds, and stir to lightly toast the spices (discard the empty cardamom pods). Add the chopped tomatoes to the pan, pour in enough water to cover the vegetables, and season well with salt and freshly ground black pepper. Bring to a simmer, then cover and leave to cook while you cook the aubergine.
Rinse the salt off the aubergine and brown the pieces lightly on a hot griddle, working in batches if necessary. Add the griddled aubergine slices to the pan with the onions, tomatoes and spices, cover and simmer until the aubergine is very soft, about 25 minutes. Taste the curry, turn off the heat then adjust the spice-level by adding natural yoghurt and crème fraîche, to taste. Sprinkle over the chopped coriander and mint and serve.