Warm the oil in a soup pot over medium heat. Add the curry paste, stirring until fragrant. Add the chicken stock, coconut milk, lemongrass, ginger, chilies, lime juice, and fish sauce and bring it to a simmer, about 5 minutes. Add the chicken strips and simmer for 15 minutes. Add the mushrooms and the 1/4 cup basil and simmer for another 5 minutes. Add salt to taste if needed. Remove the lemongrass and ginger and serve immediately, garnished with fresh basil leaves or cilantro.