The Best Thai Tom Yum Soup

Heat a large pot or Dutch oven over medium-high heat. Add the reserved shrimp shells and heads and toast for 2-3 minutes, stirring frequently, until they turn pink and fragrant. Pour in the 8 cups of water. Bring to a boil, then reduce the heat and simmer for 15-20 minutes. Strain the broth through a fine-mesh sieve into a clean bowl, discarding the solids. You should have about 6 cups of rich shrimp stock. Wipe the pot clean.

Return the shrimp stock to the pot. For the lemongrass, trim the dry ends, remove the tough outer layers, and smash the stalks with the back of a knife to release their oils. Cut them into 2-inch pieces. Add the smashed lemongrass, sliced galangal, kaffir lime leaves, and bruised Thai chilies to the stock. Bring to a simmer over medium heat and cook for 5-7 minutes to allow the flavors to infuse the broth.

Stir in the mushrooms and the optional tomato wedges. Continue to simmer for another 3-4 minutes until the mushrooms are tender.

Reduce the heat to low. Stir in the fish sauce, Thai chili paste (Nam Prik Pao), and sugar. Stir until the paste and sugar are fully dissolved. This is a critical step: do not let the soup boil rapidly from this point on.

Add the peeled and deveined shrimp to the pot. Cook for just 1-2 minutes, until they turn pink and opaque. Overcooking will make them tough. Turn off the heat immediately.

Remove the pot from the heat. Stir in the freshly squeezed lime juice and the evaporated or coconut milk (if using). Taste the soup and adjust the seasoning if necessary. It may need a bit more fish sauce for saltiness, lime juice for sourness, or a tiny pinch of sugar to balance everything out. The goal is a perfect harmony of sour, spicy, salty, and a hint of sweet.

Ladle the hot soup into bowls, making sure to get plenty of shrimp and mushrooms in each serving. Garnish generously with fresh cilantro. Serve immediately. Note that the lemongrass, galangal, and lime leaves are for aroma and are typically not eaten.