First, prep the ingredients:Wash the shrimp and remove their shells and heads. Leave the tails on. Carefully cut into their backs with a sharp knife and remove the black vein.Lightly bruise the galangal with the flat side of a knife, and cut it into slices about 1/8 inch (1/3 cm) thick. For 4 1/4 cups (1 liter) of water, use about 5-6 slices of galangal.Bruise the lemongrass with the flat side of a larger knife as well, and cut it diagonally into 3/4- to 1 1/4-inch (2-3 cm) pieces.Lightly bruise the cilantro root with the flat side of a knife too.Tear the oyster mushrooms into pieces, cut off the stem (it won’t cook through that quickly and would stay tough in the soup), and tear the larger pieces in half.If you’re using enoki mushrooms, cut off the solid base and separate them.Tear the kaffir lime leaves into 4 pieces. Since the leaf already has its two halves stacked over each other, remove the center rib too, and it will fall into 2 pieces on each side.Cut the half tomato into 6-8 smaller pieces.Crush the fresh chili peppers in a mortar until they break up, but are still partly intact.Cut the lime in half and squeeze out the juice.
Bring the stock to a boil in a medium saucepan.
Add the galangal, lemongrass, and cilantro root, and cook until the broth turns slightly greenish, about 5 minutes. Meanwhile, skim off the foam.If you’re making it with shrimp stock, it will almost certainly foam even if you already skimmed it while making the stock. But that’s not a problem, just remove it with a spoon or strainer.
Add the oyster mushrooms and the salt. Cook for about 2 minutes (the mushrooms need to soften).
Add the shrimp, the tomato, and the kaffir lime leaves. If you’re using enoki mushrooms too, add those as well.Bring it to a boil once, and turn off the heat immediately.
After taking it off the heat, add the lime juice and the fish sauce. It’s best if everyone adds the chili to their own bowl. Serve with cilantro leaves and, optionally, lime wedges.