When the pumpkin, onion and garlic cloves are soft, remove from the oven. Place the pumpkin and onion in a saucepan. Squeeze the cloves so that the garlic comes out and add to the pot. Add the thyme, chestnuts, vegetable stock and sumac. Bring to a simmer and cook for a further 10 minutes. Blitz with a hand blender. Check the seasoning.