1.Place dried mushrooms in a small bowl, cover with cold water; stand 1 hour. Drain; discard stems, slice caps thinly.
2.Meanwhile, heat oil in a large saucepan over medium heat; cook shallot, garlic and chilli, stirring, 5 minutes or until shallot softens. Add the water, stock, rind, juice and tamarind; bring to the boil.
3.Add pork, shiitake and swiss brown mushrooms to pan. Reduce heat; simmer, covered, 10 minutes or until soup is hot. Season to taste.
4.Serve soup sprinkled with onion.
Chinese barbecued pork has a sweet, sticky coating. It is available from Asian grocery stores.