Put the duck, ginger slices and smashed garlic cloves in a saucepan and cover with water. Bring to a boil and then simmer for 1 hour.
Drain the pickled mustard cabbage from the bag. Rinse or soak in water for about 10 minutes if you don't like the intense flavor.
Cut the pickled mustard cabbage into pieces. Halve the tomatoes. Place both ingredients in soup pot along with the salted plums.
Let the soup cook for another hour and season with salt and pepper. Serve.
Be sure to drain the pickled mustard cabbage when you open it and if you do not want the flavour to be too intense, soak it in warm water for a few minutes. I know you can also get fresh in some Melbourne markets.