Peel the celeriac (careful, it’s knobbly. Slice the bottom off, hold it cut side down so it doesn’t roll, and carefully trim off the exterior with a sharp knife).
Chop the celeriac into cubes, then place onto a baking sheet. Drizzle with the oil, sprinkle with thyme and toss well. Spread out onto the baking sheet in a single layer, then roast in the oven for 25 minutes or until it begins to turn golden.
Meanwhile, heat the oil in a large soup pot, then add the onion and cook for 7 minutes or until translucent. Add the garlic and cook for a further minute. Add the vegetable stock and water, bring to the boil, then reduce the heat and simmer for 5 minutes.
When the roasted celeriac is finished, add it to the pan (you may wish to reserve a few pieces for garnish), bring back to the boil and simmer for a further 3 minutes.
Transfer to a blender (you may need to do this in batches so as not to overfill) and process until smooth.
Be careful when peeling celeriac as it contains lots of knobs. Slice the bottom off, hold it cut side down so it doesn’t roll, and carefully trim off the exterior with a sharp knife.When blending the soup, use a high-speed blender and blend in batches with a towel on top to keep your hands and arms safe. Make sure the celeriac is roasted until tender and golden but not too browned. For a deep vegetable flavor, replace any water with more vegetable stock.