Heat oil in large saucepan over medium heat. Stir in onion and garlic and sauté until onion is translucent.Add ginger, red pepper, salt and lemongrass. Cook until fragrant, about 2 minutes.Add chicken to the pot and cook, stirring until chicken is white on the outside.Stir in the coconut milk, stock, fish sauce and lime juice and simmer until the chicken is thoroughly cooked and the flavors are well blended, about 10 minutes.Add bok choy, mushrooms and cilantro and simmer another 5 minutes. Taste for seasoning. Remove ginger and lemongrass stalk before serving.