Prep the flank steak. Thinly slice the steak across the grain; transfer the sliced steak to a large zip-top bag or a large mixing bowl.
Marinate. To the bag, add grated pear, soy sauce, garlic, gochugaru, ginger, brown sugar, and sesame oil; seal the bag and squish everything around to coat the meat completely with the marinade. Let sit at room temperature for 1 hour, or put in the fridge for up to 8 hours.
Heat the oil. When ready to cook, heat up 1 tablespoon vegetable oil in a grill pan or cast iron skillet set over medium-high heat.
Cook the steak. Remove one-third of the steak slices from the marinade and add them to the hot oil in a single layer; cook for 1 to 2 minutes or until browned.
Continue to cook. Flip over and continue to cook for 2 more minutes, or until browned with crisp edges. Stir occasionally. Cooking time will depend on the thickness of the steak.
Finish. Transfer the cooked pieces to a plate and repeat the above cooking steps with the remaining oil and steak.
Serve. Garnish your beef bulgogi with chopped green onions and sesame seeds, and serve.
Notes
Avoid Overcrowding: Ensure the skillet isn't overcrowded to prevent the meat from steaming instead of browning. Cook the beef in batches.
Cast Iron: For the best bulgogi, use a cast iron grill pan or skillet to mimic the effect of a hot grill. Heat it well before cooking and keep batches small to prevent crowding.
Use the Freezer: A sharp knife is key to perfect thin slices of meat, but partially freezing the steak for about 20 minutes makes slicing even easier.
Storage: Transfer leftovers to an airtight container and keep refrigerated for 3 days or freeze for up to 2 months. Reheat in a skillet over medium-low heat.