Preheat the oven to 190°C/fan 170°C/Gas 5.
Place the pumpkin or squash in a large baking tray and toss with the oil and ¼ tsp of salt. Transfer to the oven and roast for 25-30 minutes until cooked but still slightly firm.
To make the spiced tomatoes, heat the sunflower oil in a large pan and add the onion. Fry over a medium heat for 15 minutes until it is soft. Add the garlic, ginger and chilli and cook for a further 5 minutes. Toast the cumin and coriander seeds in a dry frying pan over a medium heat for a minute or so until their aromas are released, then grind in a mortar and pestle and add to the pan with the turmeric, 1 tsp of salt and ¼ tsp of black pepper. After the spices have cooked for a few minutes, add the tomatoes and sugar. Bring to a simmer, then cover and cook for 15 minutes.
Add the roast pumpkin or squash and cook for 10 minutes, pouring in the measured hot water to loosen the sauce. Taste to adjust the seasoning.
Make the yogurt sauce by combining the yogurt and garlic with ¼ tsp of salt.
To serve, transfer the pumpkin or squash to a serving plate, spoon over the yogurt sauce and sprinkle with dried mint.