Using a food processor or blender, puree 3/4 cup tuna until smooth. Mix together pureed tuna, chopped tuna, onions, ginger, vinegar, soy sauce, and sugar. Separate mixture into 6 portions and form into patties.
Arrange patties on a cooking spray coated grill and cook 2 minutes per side or until preferred degree of doneness. Let rest 5 minutes before serving.