Prep the beef in advance by freezing it for a minimum of 2 hours or overnight. This step isn’t necessary, but it will make slicing much easier.
Using a sharp knife and a steady cutting board, slice the beef into super thin pieces, then place them in a bowl for marinating.
Add the sliced onion, gochujang, minced garlic, 1/4 cup of chopped Thai basil, soy sauce, dark soy sauce, brown sugar, Thai chilies, and sesame oil to the bowl of beef slices. Mix thoroughly with your hands to ensure an even coating, then place the bowl in the fridge to marinate for at least an hour.
Once marinated, heat a cast iron skillet, Blackstone, or other cooktop. Add a drizzle of high smoke point oil to the cooking surface. Once hot, add the beef in a single layer, spreading it evenly across the surface. If using a cast iron skillet, consider cooking the beef in batches.
Cook for about 2-3 minutes, then flip and mix. Add the extra Thai basil leaves and cook for another 2-3 minutes until the beef is cooked through. Remove from the heat and serve. Cook time will vary depending on how thin your slices are, so always adjust as needed.