1. Heat the oil in a large stockpot or Dutch oven over medium heat. Add the lemon grass, ginger, and garlic and cook, stirring constantly, until aromatic, 30 to 60 seconds. Add the curry paste and cook, stirring constantly, for 30 seconds.
2. Add 1/2 cup chicken stock to the pot and stir to dissolve the curry paste. Add the remaining stock, fish sauce or soy sauce, and sugar and bring to a boil over medium-high heat. Reduce the heat to low, partially cover, and simmer to blend flavors, about 20 minutes.
3. Stir in the coconut milk, chicken, mushrooms, and lime juice. Bring back to a simmer and cook until the stock is hot and the chicken is no longer pink, about 5 minutes. Taste for seasoning and add salt if desired. Serve immediately, garnishing each bowl with cilantro and scallions.