Preheat the oven to 200°C, 400°F, Gas 6.
Peel the butternut squash, halve and scoop out the seeds. (These can be kept for toasting). Chop the butternut squash and place in a roasting tin with the onion and garlic. Toss with the rosemary, seasoning and olive oil and roast for about 25 minutes until all is golden, soft and sweet. Pop the garlic cloves from their skins and place in a blender with the squash and onion and the chestnuts and any roasting juices from the pan.
Pour in a little stock and blitz, adding the stock a little at a time. There may be some left over. Tip the ingredients into a saucepan, add the remaining stock if necessary and bring to a simmer. Adjust the seasoning and add the paprika. Meanwhile, grind the mushrooms in a mini food processor or pestle and mortar to a fine powder. Serve the soup in bowls sprinkled with the mushroom powder.
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Freezing and defrosting guidelines
Make the soup, then leave to cool at room temperature. Freeze (without garnishes or toppings) in a rigid container, leaving a bit of space for expansion, for up to 1-3 months. Reheat either from frozen or defrost in the fridge overnight. Once piping hot, add toppings or garnishes and serve.