1.Combine mince, green onions, water chestnuts, ginger, garlic, soy sauce and sesame oil in a bowl. Use clean hands to mix until well combined.
2.Line a large tray with baking paper. Spoon a heaped teaspoon of mixture into the centre of each wonton wrapper. Brush edges of wrapper with a little water and use your fingers to bring edges together so corners meet in the middle. Pinch together to seal. Place wontons on lined tray. Cover with a clean tea towel and chill until required.
3.Bring 4 cups of stock and 1 cup of water to boil in a large wok on high. Line 2 large bamboo steaming baskets with baking paper. Cut small holes in the paper. Arrange wontons in baskets in a single layer. Stack baskets, cover with lid and place over wok. Reduce heat to low and steam for 5 minutes.
4.Carefully lift steaming baskets off wok. Add cabbage and mushrooms to the stock in the wok. Swap steaming baskets around so the top basket is now on the bottom. Cover with lid and return to wok. Steam for a further 4-5 minutes, or until wontons are cooked through.
5.Remove the steaming baskets from the wok. Add the remaining stock and water, and bring to boil. Divide steamed wontons among serving bowls and ladle the broth and vegetables over. Serve immediately.