Bring the chicken broth to a boil in a saucepan. Add lemongrass and kaffir lime leaves, then add shrimp and mushrooms. As soon as the prawns are cooked after 8 to 10 minutes, remove the pan from the hob and add the remaining ingredients. Let stand for 5 minutes, then season to taste. Possibly add fish sauce or lime juice or break the chilli peppers so that they are even more spicy. The soup should taste sour, piquant and slightly salty.