1Put the lemongrass, ginger, chilli, shallots, lime leaves and stock in a large saucepan. Cover and bring to a simmer over medium heat. Simmer for 10 minutes. Remove from the heat. Cool for 10 minutes.
2Meanwhile, heat the oil in a non-stick skillet over medium heat. Add the chicken and cook for 5-6 minutes on each side until cooked through. Transfer to a cutting board, cover, and let rest for 5 minutes.
3Increase the heat under the skillet to high, add the mushrooms and cook for 3 minutes, shaking the pan occasionally, or until light golden. Remove from the heat.
4Pour the stock mixture into the blender and secure the lid. BLEND for 30-40 seconds until well combined. Strain the liquid back into the saucepan, discarding the solids. Stir in the coconut milk.
5Meanwhile, heat the oil in a non-stick skillet over medium heat. Add the chicken and cook for 5-6 minutes on each side or until cooked through. Transfer to a cutting board, cover, and let rest for 5 minutes. Increase the heat under the skillet to high, add the mushrooms and cook for 3 minutes or until light golden, shaking the pan occasionally. Remove from the heat.
6Pour the stock mixture into the blender and secure the lid. BLEND for 30-40 seconds until well combined. Strain the liquid back into the saucepan, discarding the solids. Stir in the coconut milk.
7Thinly slice the chicken across the grain and add to the soup along with the mushrooms. Bring to a simmer until hot (do not boil). Remove from the heat.
8Combine the fish sauce, lime juice and sugar and stir into the soup.
9Ladle the soup into serving bowls, and top with mint, cilantro and chili. Serve with lime wedges.