Preheat the oven to 170°C/340°F. Half the pumpkin and remove the seeds (you can keep these for roasting), then chop into wedges. Place the pumpkin on two large baking trays and drizzle over a little olive oil. In a pestle and mortar, grind the chilli and coriander seeds with a pinch of salt until finely ground. Sprinkle the spices over the pumpkin with some black pepper. Roast the pumpkin for 1 hour, or until soft and slightly caramalised at the edges.
Meanwhile, roughly chop the onion, garlic, carrot and celery. Heat a lug of olive oil over a medium heat in a large saucepan then add the vegetables and cook for 15 minutes, or until soft and sweet but not coloured.
When the squash is ready, add to the pan with the hot stock. Blend with a stick blender, adding a little more water if you like a thinner consistency.