Begin by thinly slicing beef sirloin against the grain. Chop onion and bell peppers into bite-sized pieces. Preheat a large skillet over medium-high heat.

In a mixing bowl, combine soy sauce, sesame oil, minced garlic, and sugar. Stir until dissolved. Add beef to marinade and coat. Marinate for at least 15 minutes.

Heat vegetable oil in the skillet. Add chopped onions and bell peppers, sautéing for about 5 minutes until tender and caramelized.

Push the vegetables to one side and add the marinated beef. Cook for about 5 minutes, stirring occasionally until browned and cooked through.

Once beef is cooked, stir in chopped kimchi and cook for an additional 2 minutes to infuse flavors.

Assemble by spooning the beef and veggie mixture onto toasted rolls and adding sliced cheese. Broil for about 2 minutes until cheese melts.