Pre-heat the oven to 200C. Peel the celeriac and potatoes and then along with the carrots chop into 2cm cubes. Put the chopped vegetables on a roasting tray, drizzle with olive oil and add salt and pepper then roast for 35 minutes.
When the vegetables are cooked remove from the oven and leave to cool slightly. Meanwhile fry the onions in a heavy bottomed pan in some butter for 5 minutes until soft and golden and then add the garlic and the sprigs of rosemary and cook for another 5 minutes.
Remove the rosemary sprigs and add the roasted vegetables and the vegetable stock. Mix together and allow to cool slightly and then add to a blender and blend until smooth.