Place the ribeye steak in the freezer for 30 minutes to firm up. This makes slicing easier.
Remove and slice thinly (about 2mm strips).
In a bowl, combine soy sauce, gochujang, ginger, brown sugar, apple juice (or rice vinegar), garlic, black pepper, grated apple, and sesame oil.
Add steak slices, mix well, cover, and marinate for 30 minutes (refrigerate if longer).
Heat oil in a large pan over high heat. Cook the steak for 4–5 minutes, stirring occasionally, until caramelized. Drain excess liquid if needed. Set aside to rest.
In the same pan, cook sliced bell peppers for 2–3 minutes until slightly softened. Remove from heat.
Brush melted butter on one side of each bread slice.
Place two slices butter-side down. Layer peppers, steak, sesame seeds, cheddar cheese, red onion, jalapeños, and spring onions. Top with remaining bread slices, butter-side facing out.
Cook sandwiches in a pan over medium heat for 2–3 minutes per side until golden brown and cheese melts.
Mix mayonnaise and minced garlic. Open sandwiches, drizzle garlic mayo and sweet chili sauce inside, close, and serve immediately.
This Korean Steak Sandwich is best enjoyed hot while the cheese is perfectly melted.