Dry-fry the cumin seeds and coriander seeds until fragrant. Set aside. When cooled grind finely using a pestle and mortar.
Peel the sweet potato and butternut squash and chop into bite-size pieces.
Chop the onion and garlic. Deseed the chilli, then chop finely. Peel the ginger, then chop finely.
Heat the oil in a large pot over medium heat and sauté the onion until soft. Add the mustard seeds and let them pop for a few seconds, then add the garlic, chilli, ginge and the spices (turmeric, ground cumin & coriander). Cook for a further 2 minutes.
Add the sweet potato and squash, then the chopped tomatoes and cooked chickpeas. Add the water and season with salt and pepper. Bring to the boil then gently simmer under cover for about 15-20 minutes, until the squash is tender.
Scatter the coriander on top just before serving.