Heat the oil in a large saucepan, add the black onion and cumin seeds, turmeric, chilli powder, salt, garlic and ginger and stir for 2-3 minutes, then add 200ml water.
Now add the onions and tomatoes and cook for 10 minutes stirring occasionally.
Add the carrots and potatoes, stir and then cook for another 15 minutes, covered.
Add the peas, cauliflower and remaining water and cook for a further 20-25 minutes or until both the potatoes and cauliflower soften and easily break apart.
Meanwhile, toast the walnuts in a frying pan with a tsp of olive oil and then stir them into the curry.
Add in the spinach and cook for a further 1-2 minutes until just wilted. Season to taste
