Prepare the aromatics. Trim the tough outer layers and root from the lemongrass stalks. Smash them with the back of your knife to release their aroma, then cut them into 2-inch pieces. Thinly slice the galangal. Tear the kaffir lime leaves in half, removing the central vein. Lightly bruise the Thai chilies.

In a large pot or Dutch oven, combine the chicken stock, prepared lemongrass, galangal, kaffir lime leaves, and Thai chilies. Bring to a rolling boil over medium-high heat. Reduce the heat to medium-low, cover, and let it simmer for 10-15 minutes to infuse the broth with all the aromatic flavors.

Add the mushrooms to the simmering broth and cook for 2-3 minutes until they are just tender.

Gently add the white fish chunks and squid rings to the pot. Cook for about 1-2 minutes, just until the fish becomes opaque and the squid curls up. Avoid stirring too vigorously to keep the fish intact.

Add the shrimp and mussels. Continue to cook for another 2-3 minutes, or until the shrimp turn pink and the mussels have opened. Be careful not to overcook the seafood. Discard any mussels that do not open.

Turn off the heat. Stir in the Thai chili paste (Nam Prik Pao), fish sauce, fresh lime juice, and sugar.

For a creamy Tom Yum (Tom Yum Nam Khon), now is the time to slowly stir in the evaporated milk until well combined.

Taste the soup and adjust the seasoning if necessary. It should have a bold balance of sour, spicy, salty, and a hint of sweetness. Add more lime juice for sourness, fish sauce for saltiness, or chili for heat.

Ladle the hot soup into bowls. You can either remove the large pieces of lemongrass and galangal before serving or leave them in for a more rustic presentation (they are not meant to be eaten). Garnish generously with fresh cilantro and serve immediately.