1. Prepare the meat. Partially freeze the meat. When ready, slice thinly across the grain. 2. Marinate the meat. Combine the soy sauce, rice vinegar, brown sugar, sesame oil, garlic, ginger, asian pear, and red pepper flakes in a gallon Ziplock bag, then add the beef slices. Set aside for at least two hours or overnight. 3. Cook. Next, heat a tablespoon of oil in a cast-iron skillet. Then, add one layer of beef. Let cook and char 2-3 minutes per side, then flip once. Repeat with the rest. 4. Serve. Garnish with green onions, sprinkle with sesame seeds, and serve with rice.