Preheat the oven to 180°C.
Peel the celeriac and cut into rough chunks. Break the cauliflower into small florets and chop the stem into chunks.
Spread the chunks evenly over a large oven tray, sprinkle with the paprika and roast in the oven for 20 minutes.
Meanwhile, spray the oil into a heavy-based casserole over a medium heat. Soften the onions for 7 or 8 minutes then add the garlic and stir for another couple of minutes.,
Add the celery and stock to the casserole. Bring to the boil, then simmer for 3 or 4 minutes. Add the roasted vegetables, return to the boil. Simmer for 15 minutes stirring regularly to avoid catching.
Use a handheld blender to create the consistency you prefer - you can also add more stock if you find it too thick. Season to taste and serve sprinkled with a liitle more paprika.