2 tablespoons fish sauce
2 tablespoons fresh lime or lemon juice
1 tablespoon Thai red chile paste (nam prik pao; see headnote; may substitute other chile paste such as sambal oelek)
1 stalk lemongrass, cut into 2-inch sections and crushed (see headnote)
6 thin slices (about 3/4 ounce) galangal (see headnote; may substitute fresh ginger root)
2 makrut lime leaves, torn into small pieces (see headnote; may substitute a few wide strips of lime peel)
3 cups water or chicken broth
8 ounces boneless, skinless chicken breast, cut crosswise into 1/8-inch slices (see headnote)
1/2 cup mushrooms, thinly sliced
2 cherry tomatoes, each cut in half
1 scallion, white and light-green parts, thinly sliced on the diagonal
Leaves from 1 stem cilantro
Whisk together the fish sauce, lime or lemon juice and chile paste in a small bowl.
Combine the lemongrass, galangal, makrut lime leaves and water or broth in a medium saucepan over high heat. Bring to a boil; cook for 2 to 3 minutes, then use a slotted spoon to discard the solids.
Add the chicken to the pan, making sure the pieces don’t stick together (they will begin to cook as soon as they hit the liquid). Once the liquid returns to a boil, add the mushrooms and tomatoes. Cook for 3 to 4 minutes, until the chicken has cooked through and the tomatoes have softened, reducing the heat as needed to keep the soup from boiling over.
Remove from the heat; stir in the fish sauce mixture, scallion and cilantro. Serve hot.