Tom Yum Goong (Red version)

Ingredients

350g medium sized shrimp, shell-on, head-on if possible
4 cups water (or use 3 1/2 cups unsalted chicken stock if not using shell on shrimp)
2 stalks lemongrass, bottom half only (see note)
10 pc galangal, cut into thin rounds
6 kaffir lime leaves
Thai chilies, to taste
½ cup evaporated milk
3-4 cups oyster mushrooms, straw mushrooms, or another Asian mushroom
¼ cup Thai chili paste (nam prik pao)
3 Tbsp fish sauce
¼ cup lime juice
A big handful of chopped sawtooth coriander (culantro) or cilantro

Instructions

1. Rinse shrimp well, then peel and devein them (see video for a cool deveining trick!) and place all heads and shells into a stock pot. Put the pot over medium high heat and saute the shrimp shells until stuff that sticks to the pot starts to brown slightly. It should smell like shrimp on the grill! 
2. Add water, scrape off the browned bits that are stuck to the pot, and simmer for 15-20 minutes. Then, use a slotted skimmer to remove the shells, making sure to drain off as much of the liquid as you can. 

If using chicken stock, start from here:

1. Smash lemongrass and cut into chunks. Roughly tear the kaffir lime leaves. Finely chop the chilies or pound in a mortar and pestle into a paste.
2. Bring the stock to a simmer, then add lemongrass, galangal, kaffir lime leaves and chilies (if you pounded the chilies, rinse the mortar with some of the stock to make sure you get all the chili bits into the pot). Simmer for 5 minutes. Note: The herbs are not meant to be eaten, so at this point you can either fish them out, but I like to leave some in there for garnish. Just be sure to tell your guests not to eat them!
3. Add evaporated milk and bring to boil. Add mushrooms and cook for 1-2 minutes or until mushrooms are done.
4. Add chili paste and fish sauce, and stir to dissolve the chili paste. Add shrimp meat and cook just until done, 30 seconds or so. 
5. Turn off the heat and let the soup cool down slightly, just for a minute, so it is not simmering or boiling. Add lime juice while stirring.
6. Stir in sawtooth coriander or cilantro, then taste and adjust seasoning as needed.

Recipe from Thai Hot Kitchen