01Heat oil in large saucepan over medium heat until just shimmering.
02Add lemon grass, shallots, cilantro, and 1 tablespoon fish sauce; cook, stirring frequently, until just softened, 2 to 5 minutes (vegetables should not brown).
03Stir in chicken broth and 1 can coconut milk; bring to simmer over high heat.
04Cover, reduce heat to low, and simmer until flavors have blended, 10 minutes.
05Pour broth through fine-mesh strainer and discard solids in strainer.
06Rinse saucepan and return broth mixture to pan.
07Return pan to medium-high heat.
08Stir remaining can coconut milk and sugar into broth mixture and bring to simmer.
09Reduce heat to medium, add mushrooms, and cook until just tender, 2 to 3 minutes.
10Add chicken and cook, stirring constantly, until no longer pink, 1 to 3 minutes.
11Remove soup from heat.
12Combine lime juice, curry paste, and remaining 2 tablespoons fish sauce in small bowl; stir into soup.
13Ladle soup into bowls and garnish with cilantro, chilis, and scallions.