Clean the watercress, plucking the young stems and removing the hard stems. Set aside.
Cut the pumpkin into wedges. Remove the skin and cut into smaller cubes.
Marinate the pork with 1/8 tsp of salt, rubbing evenly. Add another 1/8 tsp of salt and rub until well-combined. Set aside.
Fill a stockpot with all the water, the remaining salt, pepper, sesame oil, oyster sauce, and pumpkin cubes. Stir gently to mix the seasonings with the soup. Set the stove to medium heat. Cover the pot and let it boil for 10 minutes, or until it starts boiling.
Add all the marinated pork, stirring to separate them. Cover the pot and let the soup boil for 15 minutes.
With a spoon, cut a piece of pumpkin into half. If the spoon is able to cut it, the pumpkin is cooked and soft.
Add the lobster balls, seafood tofu and shimeji mushrooms. Stir to distribute the ingredients evenly. Cover the pot and let it cook for another 5 minutes, or until the soup comes to a rolling boil.
Taste and adjust as preferred.
Add the watercress. Cover the pot and let it boil for another minute.
Dish out the soup and serve.