Preheat the oven to 190°C.
Chop the squash and celeriac into bite-sized chunks. Cut the onion into 8 wedges.
Put the butternut squash and onion in a roasting tray, sprinkle with the thyme and add the garlic (unpeeled). Squeeze over the oil and bake for 25 minutes, until the onion and squash are tender.
Meanwhile, put the celeriac into a pan of water (about 500-600 ml), add the stock cube and bring to the boil. Simmer for 15 minutes until the celeriac is tender.
When the squash and onion are brown and tender, add them to the celeriac. Push the garlic flesh out of its skin and add to the soup.
Using a stick-blender, whizz until you get a smooth soup. Season with salt&pepper to your liking.
Serve with a dollop of low-fat yogurt or crème fraîche if dairy is not a problem for you.