Pour the stock into a large saucepan, stir in the lemon grass, garlic, coriander stalks, lime leaves, 1 chilli and ginger and simmer for 10 minutes. Remove the pieces of ginger and lemon grass.
Add the beans, water chestnuts, tofu and coconut cream and simmer until the beans are just tender. Meanwhile cook the basmati rice in a saucepan of boiling water according to the packet instructions.
Drain the rice, then add half the rice to the spring onions and half the basmati rice to the soup and heat through until piping hot.
Sprinkle the remaining coriander leaves over the soup, ladle into bowls and garnish with the remaining chilli, finely sliced. Serve with an extra dish of basmati rice for each person.