In a bowl, whisk together the soy sauce, sesame oil, brown sugar, and garlic. Toss in your thinly sliced beef, ensuring every piece is coated. Let it rest for at least 20 minutes.

Slice your hoagie rolls and lightly toast them. A smear of butter or gochujang mayo spread inside the roll provides a protective barrier against the juices.

Heat a large skillet or griddle over high heat. Add a splash of oil. Spread the beef and onions in a single layer. You want to hear that aggressive sizzle. Let it brown for 2 minutes without moving it to get those crispy edges.

Add the chopped kimchi and gochugaru to the pan. Sauté for another 3 to 4 minutes until the kimchi is softened and the liquid has mostly evaporated.

Divide the meat mixture into portions roughly the size of your rolls. Top each pile with a generous slice of provolone or mozzarella. Cover the pan for 30 seconds to let the cheese drape over the meat like a silk blanket.

Carefully scoop each cheesy portion into a roll. Top with fresh green onion and a drizzle of extra sauce if desired.