Heat the oven to 200C. Grind the coriander seeds, chilli flakes and a good pinch of salt in a pestle and mortar. Put the squash in a roasting tray and drizzle with olive oil. Sprinkle over the spice mixture and mix to coat well.
Roast in the oven until the squash is soft and golden at the edges.
Meanwhile, fry off the onions until soft, add the curry powder, lentils and stock and simmer. Add the squash when it is cooked and simmer until the lentils are cooked, about 20-30 mins. Add a pinch of garam masala and blend well with a stick blender until smooth.
(Use vegetable stock instead of chicken if you want this to be a vegetarian dish)