In a large zip-top bag, combine Wagyu beef, onion, garlic, ginger, soy sauce, brown sugar, vinegar, sesame oil, and black pepper. Seal and shake to combine. Marinate in the fridge for at least 1 hour (or overnight).

Heat 1 inch of oil in a cast iron skillet to 350°F. Fry wonton wrappers in batches, flipping once, until lightly golden. Drain on paper towels and sprinkle with salt while hot. Discard all but 1 tablespoon of oil once cooled.

Return skillet to medium heat. Add kimchi and sauté until soft and caramelized, about 10 minutes. Transfer to a plate.

Drain marinated beef and cook in batches over high heat until browned, about 4 minutes. Add oil as needed if pan gets dry. Set beef aside and wipe excess oil from skillet.

Combine kimchi and cooked beef. Stir 2 tablespoon gochujang into cheese sauce and keep warm.

Preheat oven to 350°F. In your skillet or baking dish, layer wonton chips, beef/kimchi mix, cheese sauce, and shredded cheese. Repeat layers until ingredients are used up.

Bake until cheese is melted and nachos are heated through, about 5 minutes.

In a bowl, whisk sour cream with lime juice. Drizzle over hot nachos along with extra gochujang. Top with sesame seeds, cilantro, scallions, and chile slices if using. Serve immediately.