Combine soy sauce, sugar, oil, gochujang, and garlic in a small saucepan; cook over medium-high heat 3 minutes or until reduced by half.
Preheat grill pan over medium-high heat. Add patties, and cook 5 to 6 minutes per side or until no longer pink, brushing burgers with sauce as they cook.
Place patties on buns, and top with kimchi and, if desired, onions.
Charred Pineapple
Preheat a grill pan over medium-high heat. Sprinkle pineapple with brown sugar, and cook 4 to 5 minutes per side or until charred.