Peel and finely chop the onion. Slice the roots off the celeriac at an angle, so you take the roots off but leave as much flesh as possible. Peel the rest of the celeriac. Roughly chop it. Warm a large pan for 1 min over a medium heat. Add 1 tbsp oil and the chopped veg. Season. Cover. Sweat over a low heat for 10 mins.
While the veg sweat, peel and grate the garlic and ginger. Finely chop the chilli, flicking out the seeds for less heat. Finely chop the coriander stalks. Zest and juice the lime.
Stir the garlic, ginger, chilli, coriander stalks and lime zest into the veg. Pour in 900ml boiling water (or hot homemade stock). Cover, turn the heat up and bring to the boil. Turn the heat down a little and simmer for 15 mins till the veg are soft.
Ladle the veg into a blender and blitz till smooth (or use a hand-held blender in the pan). Swirl in the lime juice. Taste and add more salt and pepper if it needs it. Serve in warm bowls topped with the coriander leaves.