Peel and chop the sweet potato. Cut the spring onions into rings, halve the mushrooms and tomatoes.
Let the sweet potato cook in the hot broth for 5 minutes. Add coconut milk, lemongrass, ginger, 1 tablespoon lime juice, tomatoes and mushrooms and simmer for another 5 minutes.
Soak the spring onions briefly, season with salt and serve sprinkled with coriander.