Heat a little oil in a hot wok and saute the ginger and prawns until fragrant. Add the tomato and stir-fry until soft. Add the water and bring to a boil.
Add the oyster mushrooms and simmer for ten minutes. Add the bean sprouts and simmer two minutes more. Season with salt and sesame oil and serve.
Tip:
Tiny dried prawns or shrimp can be found in Asian shops.