Place 3/4 cup tuna in a food processor; process until smooth. Combine pureed tuna, remaining chopped tuna, onions, and next 4 ingredients (through sugar). Divide mixture into 6 equal portions using wet hands, shaping each into a 1/2-inch-thick patty.
Place patties on a grill rack coated with cooking spray; grill 2 minutes. Turn patties over; grill 1 minute or until desired degree of doneness. Remove from grill; let stand 5 minutes.