Place the pork hocks, galangal, kaffir lime leaves and lemongrass in a pot with the stock. Bring the pot to a simmer and cook the hocks until they are tender. This may take a couple of hours.
While the hocks are cooking, make sure to skim off any fat and scum that may rise to the top of the pot. If the liquid evaporates too much, add more water or stock to ensure the hocks are staying covered.
When the pork hocks are tender, add the chillies, shallot, culantro and tomatoes. Cook for 5 minutes.
Season the soup with the fish sauce, lime juice and tamarind juice. Just before serving, adjust the seasoning, it may need more stock, lime juice or fish sauce.