Combine 4 litres water; 1 cup carrots, sliced; 1 cup cabbage, sliced; 1/2 cup celery stalks, sliced; 1 cup white radish, sliced; and 1.5 tsp salt in a saucepan and boil for 1 hour, or until the liquid has reduced by half.
Strain, and set aside.
Pound the coriander roots, lemongrass, asafetida and chili to a rough paste using mortar and pestle. Alternatively, process in a blender with a few drops of water.
Boil the vegetable stock in a saucepan over full heat. Drop in the kaffir lime leaves, mushroom, and tomato and cook for 2 or 3 minutes.
Add the spice paste, reduce the heat to a simmer, add the tofu and sorrel and cook for another 2 minutes.
Serve: Divide the extra chili, the soy sauce, lime juice and chopped coriander into large deep serving bowls and pour the boiling soup over. Serve with hot steamed rice.