If you want a soup with less fat, it is possible to substitute light coconut milk for one or both cans of regular coconut milk. Fresh lemon grass can be omitted, but the soup will lack some complexity. Don't be tempted to use jarred or dried lemon grass—their flavor is characterless. If you want a spicier soup, add more red curry paste to taste. For a more substantial meal, serve the soup over 2 to 3 cups of cooked jasmine rice. The soup can be prepared through step 1 up to one day ahead of time and refrigerated, but it should be completed immediately before serving, as the chicken and mushrooms can easily overcook.

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

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

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