In a large mixing bowl, combine the sliced beef sirloin with soy sauce, gochujang, sesame oil, minced garlic, salt, and pepper. Mix well and marinate in the refrigerator for at least 30 minutes.

Prepare the vegetables by slicing the onions and bell peppers.

Heat 1 tablespoon of vegetable oil in a large skillet over medium-high heat. Add the sliced onions and bell peppers, sauté for 3-4 minutes until softened and caramelized, then set aside.

Add the remaining tablespoon of vegetable oil to the skillet. Once heated, add the marinated beef in a single layer and cook for 2-3 minutes on each side until browned.

Stir in the chopped kimchi and cook for an additional 1-2 minutes until heated through.

Toast the hoagie rolls in a preheated oven at 350°F for about 5 minutes.

Layer the beef and kimchi mixture onto the toasted hoagie rolls and top with provolone cheese.

Return sandwiches to the oven for another 2-3 minutes to melt the cheese.

Garnish with chopped green onions and sesame seeds if desired. Serve hot.