Shell the prawns. Keep them cool and use the heads and shells to make the flavored broth. Peel and finely chop the onions. Peel and press the garlic cloves.
In a casserole dish, heat the oil and brown the shrimp shells and heads, onions and garlic over high heat. Add all the spices (dried lemongrass, ginger and chilli), salt and pepper, then pour in 1.25 l of water and the fish stock. Mix and simmer gently for about 30 minutes. Filter, taste and, if necessary, adjust the seasoning.
Squeeze the lemon and collect the juice. Peel the carrots and, using a peeler, cut them into strips (or julienne). Wipe the mushrooms, slice very fine. Boil the fragrant broth, add the carrots, cook for 5 minutes then, at the last moment, put the prawns and mushrooms in the broth, just to heat them (the mushrooms should remain crunchy).
Serve very hot in bowls, sprinkle with lemon juice, sprinkle with chopped coriander and add a few drops of nuoc-mâm, according to taste.
