Cook the rice in boiling salted water for about 10 minutes, until tender, then drain and set aside.
Meanwhile, heat the stock in a large pan, add the lemongrass, galangal or ginger, lime leaves and chillies and simmer for 5 minutes. Add the chicken and mushrooms and simmer for a further 5 minutes. Stir in the tomatoes, lime juice, fish sauce and coconut cream and simmer for 5 minutes more. Scatter over the coriander and serve each portion with a little cooked rice spooned in.