Begin by slicing the beef sirloin thinly against the grain to ensure tenderness, then set it aside. Next, chop the onion and bell peppers into uniform pieces.

In a medium mixing bowl, combine soy sauce, sesame oil, minced garlic, and sugar to create a savory marinade. Add in the sliced beef and let it marinate for at least 15 minutes.

Heat a splash of vegetable oil in a large skillet over medium-high heat. Sauté the chopped onion and bell peppers for about 5 minutes until tender and slightly caramelized.

Push the vegetables to the side, add the marinated beef, and cook for approximately 5 minutes until browned and cooked through.

Mix in the chopped kimchi and cook for an additional 2 minutes, allowing the flavors to meld.

Preheat your broiler and prepare your sub rolls. Spoon the beef mixture into the rolls and top with cheese. Broil for about 2 minutes until bubbly and melted.

Allow to cool slightly before serving. Optionally drizzle with extra soy sauce and cut in half for easy handling.