Thinly slice beef sirloin against the grain, chop onion and bell peppers into bite-sized pieces.

Combine soy sauce, sesame oil, minced garlic, and sugar in a bowl, add beef, and marinate for at least 15 minutes.

Heat a skillet over medium-high heat, add vegetable oil, and sauté onion and bell peppers for about 5 minutes.

Push veggies to one side of the skillet, add marinated beef, and cook for 5 minutes until browned.

Mix in kimchi with beef and veggies, sauté for an additional 2 minutes.

Assemble cheesesteaks on toasted hoagie rolls with bulgogi mixture and provolone cheese, broil for about 2 minutes until cheese is bubbly.