1 tsp. vegetable oil
3 stalks lemon grass, sliced thin
3 large shallots, chopped
8 sprigs fresh cilantro leaves, chopped
3 tbsp. fish sauce
4 c. chicken broth
2 14 oz. cans coconut milk
1 tbsp. sugar
½ lb. white mushrooms, cut into 1/4″ slices
1 lb. pork tenderloin, halved lengthwise, sliced into 1/8″ thick pieces
3 tbsp. fresh lime juice
2 tsp. red curry paste
Heat oil in large saucepan over medium heat. Add lemon grass, shallots, cilantro, and 1 tbsp. fish sauce; cook, stirring frequently, until just softened, 2 to 5 minutes. Stir in chicken broth and 1 can coconut milk; bring to simmer. Cover, reduce heat to low, and simmer 10 minutes. Strain broth, return to pan.
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 pork and cook, stirring constantly, until no longer pink, 1 to 3 minutes. Remove soup from heat.
Combine lime juice, curry paste, and remaining 2 tablespoons fish sauce in small bowl; stir into soup.