Step 1 In a medium bowl, combine the beef, soy sauce, rice wine, sugar and salt. Use a fork, chopsticks or spatula to stir together the ingredients clockwise (do not stir in the opposite direction) until mixture is smooth like a paste. Add water in 3 stages, stirring clockwise until each addition is well-combined and liquid is emulsified. Stir in the oil in same manner.
Step 2 Stir in leeks and ginger until well combined.
Step 3Use the filling immediately, or wrap and chill until ready to use, no more than 1 day.