If you don't have your butcher slice the sirloin. Pop the sirloin in the freezer for 20 minutes to make slicing easier. Be sure to slice against the grain – this will produce shorter muscle fibers, giving you a more tender bite.
In a large bowl, Garlic (3 clove), Yellow Onion (1/2), Scallion (3), Low-Sodium Soy Sauce (1/2 cup), Sesame Oil (2 tablespoon), Dark Brown Sugar (3 tablespoon), Freshly Ground Black Pepper (1/8 teaspoon) and Gochujang (1 tablespoon).
In a large zip-top bag, add the Sirloin Steak (2 pound) and mix well, making sure that each piece is thoroughly coated. Cover, refrigerate and allow to marinate for 2 hours.
To caramelize onions, Melt the Unsalted Butter (2 tablespoon) and Olive Oil (2 tablespoon) in a large saute pan over medium heat. Add the Yellow Onion (3). Cook, stirring occasionally for 35 to 40 minutes until soft and caramelized.
Add the Balsamic Vinegar (1 tablespoon) and season with Kosher Salt (to taste) and pepper to taste. Toss to combine and set aside.
Using a heavy-bottomed skillet over high heat, add a ¼ of the beef along with some of the marinade and toss to cook for about a minute.
Form the beef into the approximate length of the hoagie roll. Top with a ¼ of the caramelized onions and ¼ of the grated Provolone Cheese (12 ounce). Add a tablespoon of the reserved marinade. Cover the pan with a lid and cook about a minute.
Slice your Hoagie Roll (4). Don't cut all the way through, you want it to open like a book.
Using two spatulas, one on either side, scoop up the beef and place inside the hoagie rolls. Sprinkle with some Crushed Red Pepper Flakes (to taste) if desired. Serve immediately and enjoy!