Place mince, onion, cornflour, soy sauce, rice wine, ginger and egg in a large bowl. Mix to combine and set aside.
Heat stock in large saucepan on high. Add leek and tomato and boil for 2 minutes. Reduce heat to medium. Drop teaspoons of mince mixture into gently simmering soup and cook for 5 minutes. Serve soup scattered with coriander leaves.