Cut the ginger into slices. Clean and halve the mushrooms. Cut the lemongrass and spring onion into pieces about 3 cm long.
Let the vegetable stock with ginger, spring onion and lemongrass cook for about 5 minutes. Add the prawns, mushrooms, chili paste and oyster sauce. Let simmer for another 10 minutes.
Add the coconut milk and stir. Season the soup with lime juice, salt and pepper. Let it stand for a few minutes. Roughly chop the coriander leaves and pour over the soup.