For Tom Ka Gai, first heat peanut oil in a saucepan and fry the curry paste in it until hot. Add chicken stock, coconut milk, lemongrass, ginger, galangal, lime leaves, chili, lime juice and fish sauce and simmer for 10 minutes. Add the chicken breast strips and simmer for another 15 minutes. Then add mushrooms and some Thai basil, season with sea salt if desired and let simmer again for 5 minutes.
Put the rice vermicelli in a bowl, pour the soup over it and garnish the Tom Ka Gai with fresh coriander.