Boil the clear broth and add the chopped lemon grass, kaffir lime leaves and coriander roots. Cover the pot until it begins to boil. Add the tomatoes, mushroom and fish sauce and leave to cook. Add he prawns, wait for the broth to boil, remove the pot from the heat. Put chili paste and lime juice into a bowl, spoon stir until mixed well, then add he small chilies. Pour the Tom Yum soup into the bowl and stir gently until well blended. At this point, if required add additional spices to taste. Sprinkle the coriander leaves and serve while hot.