Heat half the oil in a frying pan on a medium heat and sauté the aubergines and okra for 5-10 minutes until they start to colour.
Meanwhile, heat the other half of the oil in a medium saucepan and add the fennel, nigella and cumin seeds and stir fry for 30 seconds. Then add the ginger, garlic and green chillies and cook for a further 30 seconds taking care not to let it burn.
Add the turmeric and immediately add the tomato purée and whizzed tinned tomatoes. Allow the mixture to simmer for 4-5 minutes then add the aubergines, okra and potatoes. Cook for a couple of minutes until the aubergines, okra and potatoes are fully covered in sauce, then add the stock and cook uncovered for 10-15 minutes on a medium heat.
Stir through the coriander, season and serve.