Half-fill a medium saucepan with water and bring to the boil. In another medium saucepan, bring the stock and coconut milk to the boil. Once the mixture is bubbling away, add the galangal, lemongrass, chillies and coriander roots or stalks and cook over a medium heat for about 10 minutes, until the flavours come through and the soup has turned a slightly yellow colour.
Now add the oyster mushrooms, tofu, chilli paste, lime juice, soy sauce, sugar and salt. Cook for a further minute, then add the tomatoes and coriander leaves. Reduce the heat to a simmer and cook for a further minute, until the tomatoes have softened.
Cook the noodles in the boiling water for 20 seconds, then drain. Divide the noodles between two bowls, then pour over the soup. Garnish with the coriander leaves and serve immediately.