1.Separate stems from caps of the shiitake mushrooms; reserve stems. Finely chop a quarter of the shiitake caps; slice remaining shiitake caps thinly. Coarsely chop white section of green onion. Thinly slice green section; reserve green section in small bowl.
2.Combine shiitake mushroom stems, white section of green onion, brown onion, carrot, garlic, ginger, star anise, peppercorns, sauce, wine, coriander roots, sugar and the water in large saucepan; bring to the boil. Reduce heat; simmer, uncovered, 45 minutes. Strain stock through muslin-lined sieve or colander into large heatproof bowl; discard solids. Return stock to same cleaned pan; bring to the boil. Reduce heat; simmer, covered.
3.Meanwhile, separate bases and leaves of buk choy. Finely chop half of the bases; discard remaining bases. Combine chopped bases with finely chopped shiitake mushrooms, extra ginger, extra garlic, water chestnuts, egg white, breadcrumbs, coriander leaves and oyster sauce in large bowl. Make dumplings by placing 1 heaped teaspoon of the mushroom mixture in centre of a wonton wrapper; brush around edges with a little water. Fold wrapper in half diagonally; pinch edges together to seal. Repeat with remaining mushroom mixture and wrappers.
4.Add sliced shiitake and enoki mushrooms to simmering stock; cook, uncovered, 5 minutes. Add dumplings; cook, uncovered, 5 minutes or until dumplings are cooked through. Stir in buk choy leaves.
5.Divide soup among serving bowls; top with reserved sliced green onion.
Vegetarian oyster sauce can be found in most supermarkets and Asian food stores.Note