Rinse dried Chinese yam under running water; Soak it in water for 3 hours; Remove and drain it.
Rinse dried longan under running water; Soak it in water for 3 hours; Remove and drain it.
Rinse chestnuts under running water; Drain them; Make a small incision in the skin with a paring knife to avoid their explosion; Fill a deep pan with water; Bring to the boil over a high heat; Add the chestnuts; Cook over a high heat for 6 minutes; Remove them with a wire skimmer; Turn off the heat; Leave them to cool a bit; Peel off the tough shells and the papery thin skin underneath while the chestnuts are still hot.
Rinse pumpkin under running water; Drain it; Slice off the stem; Cut the pumpkin in half with a cleaver; Scrape out the seeds; Peel it with a vegetable knife; Cut it into chunks.
Rinse, drain and slice ginger.
Rinse lean pork under running water; Pat-dry with kitchen paper; Cut it into chunks.
Discard the water in the deep pan; Fill it with water again; Add the ginger; Bring to the boil over a high heat; Scald the lean pork all over in a colander with the boiling ginger water for 3 minutes; Immediately remove it; Rinse it under running water till cool; Drain it; Turn off the heat.
Fill a stockpot with 2.8L water; Bring to the boil over a high heat; Add all the ingredients; Bring to the boil over a high heat again; Turn to a low heat; Cook over a low heat for 2 hours; Turn to a high heat; Cook over a high heat for 30 minutes; Stir seasonings into the soup; Turn off the heat; Arrange the soup in serving bowls; Ready to serve.