Prepare Ingredients: Slice the beef sirloin thinly against the grain, chop onion and bell peppers into strips.

Marinate Beef: Whisk together soy sauce, sesame oil, minced garlic, and sugar. Marinate beef for at least 15 minutes.

Sauté Vegetables: Heat vegetable oil over medium-high heat, add onions and bell peppers, sauté for about 5 minutes.

Cook Beef: Push vegetables to the side and add marinated beef. Cook for about 5 minutes until browned.

Combine with Kimchi: Add chopped kimchi to skillet, stir to combine, and cook for an additional 2 minutes.

Assemble Cheesesteaks: Toast hoagie rolls, spoon bulgogi mixture onto rolls, add cheese, and broil for 2 minutes.