If your beef is not pre-sliced, partially freeze the sirloin or ribeye for 30-60 minutes to make it firm enough to slice very thinly against the grain. Pat the sliced beef dry with paper towels.

In a large mixing bowl, combine the soy sauce, brown sugar, sesame oil, mirin, grated fresh ginger, minced garlic, grated or pureed Asian pear (or apple), water (or beef broth), and black pepper. Whisk until the sugar is dissolved.

Add the thinly sliced beef and sliced yellow onion to the marinade. Use your hands to thoroughly mix, ensuring every piece of beef is coated. Cover the bowl and refrigerate for at least 2 hours, or preferably overnight for deeper flavor.

Heat a large skillet or wok over medium-high heat. Add a tiny bit of oil if your pan isn't non-stick. Once hot, add a single layer of marinated beef (without overcrowding the pan) and some of the sliced onions from the marinade. Do not pour in excess marinade liquid.

Cook for 2-3 minutes per side, stirring occasionally, until the beef is browned and cooked through. It cooks quickly due to its thinness. Remove the cooked beef to a serving platter.

Repeat with the remaining beef in batches until all the bulgogi is cooked. This ensures the beef sears rather than steams.

Garnish the cooked bulgogi with thinly sliced green onions and toasted sesame seeds. Serve immediately with warm cooked rice. Optionally, serve with fresh lettuce leaves for wrapping the beef, and kimchi as a side dish.