Heat the oven to 180C/160C fan/gas 4. Peel the pumpkin and cut in half and remove the seeds with a spoon. Cut the halves into 2cm chunks and put into a bowl.
Put the garam masala, and 1 tsp each of the coriander and cumin into a small bowl and mix with 2 tbsp of the oil and season. Drizzle over the pumpkin and toss well to coat in the spiced oil.
Transfer the pumpkin chunks to a baking tray, spread out evenly and roast for 40-45 mins, turning halfway through cooking, until the pumpkin is very soft when pierced with a fork. Leave to cool on the tray for a few minutes.
Heat the remaining 1 tbsp olive oil in a large saucepan and fry the onion with a pinch of salt for 10 mins until soft. Add the ginger, garlic and remaining spices and chilli flakes, and fry for a few more minutes until fragrant. Pour in the stock and bring to a gentle simmer.
Add the soft pumpkin to the stock pan. Remove from the heat and blitz the soup with a hand blender until creamy and smooth. Season to taste, adding extra chilli or garam masala if you like. Put back over a low heat and stir in the cream. Bring to a gentle simmer, then serve.