Prepare the meatballs, by mixing beef with onion, garlic, cornstarch, and seasoning in a food processor. Bind with sufficient egg white to make a firm mixture. With wet hands, roll into tiny, bite size balls and set aside.
Drain the mushrooms. Add the soaking liquid to the stock. Trim off and discard the stalks. Slice the caps finely and set aside.
Heat a wok or large pan, add the oil. Fry the onion, garlic and ginger for 2-3 minutes to bring out the flavor. Be careful not to burn.
When onion is soft, pour in the stock. Bring to boil, then stir in soy sauce and mushroom slices and simmer for 10 minutes.
Add the meatballs and cook for a further 10 minutes.
Before serving, remove the ginger. Stir in the shredded curly kale, spinach, or Chinese cabbage. Heat through 1 minute only. Do not overcook.
Ladle soup into warmed bowls and serve piping hot.