Tip the lentils into a bowl and cover with cold water. Set aside to soak while you prepare the veg.
Peel and finely chop the onion. Halve the squash, scoop out the seeds, then peel it. Chop into 1cm chunks. Peel and chop the sweet potatoes to match.
Warm a large pan or casserole over a medium-low heat. Add 2 tbsp oil, the onion, squash and sweet potato with a pinch of salt and pepper. Cook, stirring every so often, for 12-15 mins till the veg start to soften.
While the veg cook, peel and grate the garlic, ginger and turmeric onto a plate. Halve the chillies. Scoop out the seeds and pith for less heat. Finely chop them.
Stir the garlic, ginger, turmeric and chillies into the veg. Add 2 tsp garam masala and 1 tbsp cumin seeds. Crumble in the stock cube. Tip in the tin of chopped tomatoes. Fill it with water (approximately 400ml) and add that to the pan as well.
Drain the lentils and stir into the pan. Bring to the boil. When boiling, turn the heat down, cover and simmer for 25 mins till everything is tender and the curry has thickened. Stir now and then to stop it from sticking. Add a splash more water if needed.
Taste and add a pinch more salt and pepper if you think it needs it. Ladle the curry into warm bowls and serve garnished with coriander leaves.