Use a knife to run through the back of the shrimp to devein. Use kitchen shears to trim off the tentacles. Leave the heads and the shells intact. If you choose to take off the head, you can use the head to make the stock below (use water instead of chicken broth)
In a large pot, preheat 1 Tbsp of cooking oil. Add in the shallots and garlic and stir fry for about 1 minute. Add in the fresh galangal, lemon grass, kaffir lime leaves, red chili and stir fry for another 1 minute. Add the chicken broth and bring to a boil. If you are using the shrimp's head to make a stock, add it in at this point. Bring it to a boil and let it simmer for 10 minutes. Use a slotted spoon to remove the heads and discard them. Skim off some scums that might formed from the boiling the shrimp heads. Add in the oyster mushrooms and let it cook for 1 minute. Add in the shrimp and let them cook for 2 minutes or so or until the shrimp is cooked through
It's time to season the soup. Stir in the fish sauce, lime juice, and sugar. Have a taste. It should be sour, spicy, savory, with just a hint of sweetness. Add more fish sauce and/or lime juice as needed
When ready to serve, garnish with some fresh cilantro leaves and serve immediately