- Thoroughly combine beef, onion and 2 Tbsp. soy sauce; shape into 4 thick oval patties.
- Cook patties in preheated large skillet over medium heat 4 minutes. Turn patties over; cook 3 minutes longer, or until done.
- Meanwhile, combine 2 cups water, remaining 2 Tbsp. soy sauce, mustard and pepper.
- Remove patties to serving platter; keep warm.
- Drain off drippings from pan; return 3 Tbsp. Blend in flour and cook over medium heat 1 minute, stirring constantly. Gradually stir in soy sauce mixture. Bring to boil; cook and stir until sauce thickens.
- Serve with patties.