thai-style chicken soup

1 teaspoon vegetable oil
3 lemongrass stalks, tough outer leaves removed, bottom 5 inches halved lengthwise and sliced thin crosswise
3 shallots, chopped (about 1/2 cup)
8 sprigs fresh cilantro, chopped
3 tablespoons fish sauce
4 cups low-sodium chicken broth
2 (14-ounce) cans coconut milk, well shaken
1 tablespoon sugar
1/2 pound white mushrooms, cut into 1/4-inch slices
3 boneless, skinless chicken breasts (5-6 ounces each), halved lengthwise and sliced on the bias into 1/8-inch-thick pieces
3 tablespoons fresh lime juice from 2 to 3 limes
2 teaspoons Thai red curry paste

Garnish:
1/2 cup fresh cilantro
2 serrano chiles, sliced thin
2 scallions, sliced thin on the bias
1 lime, cut into wedges

Heat the oil in a large saucepan over medium heat until just shimmering. Add the lemongrass, shallots, cilantro, and 1 tablespoon of the fish sauce; cook, stirring frequently, until just softened, 2 to 5 minutes (the vegetables should not brown). Stir in the broth and 1 can coconut milk; bring to a simmer over high heat. Cover, reduce the heat to low, and simmer until the flavors have blended, 10 minutes. Pour the broth through a fine-mesh strainer and discard the solids in the strainer. Rinse the saucepan and return the broth mixture to the pan.

Return the pan to medium-high heat. Stir the remaining can of coconut milk and sugar into the broth mixture and bring to a simmer. Reduce the heat to medium, add the mushrooms, and cook until just tender, 2 to 3 minutes. Add the chicken and cook, stirring constantly, until no longer pink, 1 to 3 minutes. Remove the soup from the heat.

Combine the lime juice, curry paste, and remaining 2 tablespoons fish sauce in a small bowl; stir into the soup. Ladle the soup into bowls and garnish with the cilantro, chiles, and scallions. Serve immediately with lime wedges.