Defrost the prawns. Bring the water to a boil, dissolve the mushroom broth in it. Soak the mushrooms with 150 g of it. After 30 minutes, gently squeeze the broth and return it to the saucepan. Quarter the mushroom caps and discard the stems.
Wash the fresh lemongrass, remove the hard stalk at the bottom, remove the brown and withered leaves and only use the white to light green parts. Cut this into pieces approx. 8 cm long. Remove the outer, green leaves if necessary. Gently crush the lower half of the pieces with a hammer. The stem should remain intact. Wash the carrot, cut off both ends, peel and slice it into approx. 3 mm thick slices with a corrugated plane.
Wash and peel the fresh ginger and galangal, finely grate the ginger and cut the galangal crosswise into thin slices. Cap the garlic cloves at both ends, peel and squeeze with a garlic press. Wash the kaffir lime leaves and use them whole.
Add lemongrass, carrot, ginger, galangal, garlic and kaffir lime leaves to the simmering broth. Simmer gently for 20 minutes with the lid on.
Wash the tomato, remove the stem, peel it, quarter it lengthways, remove the green stem and the grains. Core the quarters and cut in half crosswise. Wash the red peppers and remove the stem. Depending on how sharp you want, remove or leave the grains. Cut the pod diagonally into pieces approx. 6 mm wide. Rinse the mung seedlings and shake dry in a sieve. Distribute the seedlings in the serving bowls.
Stir the fish sauce, spring roll sauce and lime juice into the broth. Add tomatoes and mushrooms and simmer for 5 minutes. Reduce the heat and stir the coconut milk into the broth. Add the prawns and simmer until they are pink. Distribute the finished soup on the serving bowls, garnish and enjoy warm.