Start by slicing the beef sirloin into thin strips and set aside. Chop the onion and bell peppers into bite-sized pieces.

In a medium bowl, whisk together soy sauce, sesame oil, minced garlic, and sugar. Add beef strips and marinate for at least 15 minutes.

Heat a skillet over medium-high heat and add a tablespoon of vegetable oil. Sauté onions and bell peppers for about 5 minutes until tender.

Push sautéed vegetables to one side of the skillet, add marinated beef strips, and cook for about 5 minutes until browned. Mix in vegetables.

Add chopped kimchi to the skillet, stir everything together, and cook for an additional 2 minutes.

Slice hoagie rolls in half, optionally toast them, and load with beef, vegetables, and kimchi mixture. Top with cheese and broil for about 2 minutes.

Remove from oven, let cool slightly, and serve warm with optional garnishes.