Heat oil for sautéing in a large pot and sauté pepper, onion, Thai chilies, ginger, garlic, and mushrooms. Once softened, after about eight to 10 minutes, add the chicken and keep sautéing.

Once nicely browned, add the stock. Place the stems from the basil and cilantro into cheesecloth and add to soup. Puree the lemon grass in a blender, place it in cheesecloth, and add to soup. Let simmer for 45 minutes.

Add coconut milk, lemon juice (squeezed from the lemon), the lemon itself, and the scallions. Add about half the torn basil and cilantro to the soup. Let simmer for another 45 minutes, without bringing to a hard boil. Add soy sauce to taste at the end.

Before serving, let soup sit for about a day so all the flavors can come together.