Halve your shallots lengthwise. Trim the rooty end. Peel. Thinly slice.
Get a large pot hot. Gloss with oil. Add your shallots. Season well. Sizzle over medium-low heat till tender and glossy, about 7 mins.
Finely chop the garlic. Swirl into the onions along with the fennel seeds. Cook a minute. Fold in the curry powder.
Pour in your stock or water. Bring to the boil. Lower heat. Simmer while you prep the roots.
Carve the peel off your celeriac using a knife. [Use the remaining celeriac to make a speedy veg stock.*] Coarsely grate it and the carrot. Add to the simmering broth.
Grate in the lime zest. Add a good squeeze of juice. Taste. Season as needed.
Rinse your watercress. Pluck out any woody stalks. Pour half the broth into your bowls. Pile a mound of watercress on top. Add more broth. Eat, marrying the watercress into the broth with each bite.