Place the tamarind piece in a small bowl and add in about 1/2 cup of water and squeeze the tamarind with your hand to extract the juice. Set aside. Trim off 1-inch of the woody end on the bottom and the skinny stalks on top of the lemongrass stalks. Cut each stalk into half and smash them with heavy objects to release flavor
In a large heavy-bottom pot, preheat the oil. Add in shallots, lemongrass, and garlic and saute until fragrant, about 2 minutes. Add in the pork strips and cook until they start to turn color. Add in 4 cups of water. Bring to a boil then lower the heat and let it simmer until the pork is cooked through and tender, about 15 minutes. Season with salt and have a taste and adjust seasoning as needed
When ready to serve, discard the lemongrass stalks, ladle the soup over serving bowl and garnish with some coriander leaves