Soak the chillies in boiling water. In the meantime, heat up the oil a deep frying pan or saucepan.
Add the mustard seeds and wait until they pop. Add the cumin, coriander, cinnamon, bay and cloves. Add the butternut squash to the pan, stirring on high heat then lower the heat to medium.
Remove the chillies from the boiling water, snipping off the stems and seeds. Place them in a blender with the tamarind paste and a little water. Blend until you have a watery paste. Add this chilli tamarind paste to the pan and stir.
Add the garlic, then grate in the ginger and turmeric. Add the green tomatoes, followed by the salt and coconut milk.
Cook on a medium to low heat until the butternut squash is tender. Finally, add the fresh coriander leaves and serve with rice.