Slice button mushrooms and ginger. Chop bamboo shoot. Heat oil in a non-stick pan. Add ginger and sauté for a minute.
Add mushrooms and bamboo shoot and sauté for a minute. Add chicken stock and let it cook. Grind prawns with salt and black pepper powder to a fine paste. Transfer prawn paste into a bowl.
Add cornflour and egg white and mix well. Drop in small portions of prawn paste with a spoon into the boiling stock. Cook till the prawn balls are done. Tear the spinach leaves and add to soup and stir lightly.
Add salt and crushed peppercorns and mix. Chop spring onion greens and add to soup. Mix well and cook for 2 minutes. Serve hot.