If your beef is not already thinly sliced, partially freeze it for 30-45 minutes to make it easier to slice thinly against the grain. Slice into pieces about 1/8 inch thick.

In a large mixing bowl, combine the soy sauce, brown sugar, sesame oil, mirin, rice vinegar, grated Asian pear (or apple juice), minced garlic, grated ginger, black pepper, and grated white onion. Whisk until the sugar is dissolved and the marinade is well combined.

Add the thinly sliced beef to the marinade. Toss gently to ensure all pieces are coated. Cover the bowl and refrigerate for at least 2 hours, or preferably 4-6 hours, for the best flavor. You can also marinate overnight.

When ready to cook, heat a large non-stick skillet or wok over medium-high heat. Add a tiny drizzle of neutral oil if your pan isn't perfectly non-stick, but the sesame oil in the marinade usually suffices.

Working in batches to avoid overcrowding the pan, add a single layer of marinated beef. Do not pour in excess marinade. Cook for 2-3 minutes per side, or until the beef is nicely caramelized and cooked through. Overcrowding the pan will steam the meat instead of searing it, making it less tender and flavorful.

Transfer the cooked bulgogi to a serving platter. Repeat with the remaining beef until all batches are cooked.

Garnish the cooked bulgogi with toasted sesame seeds and chopped green onions.

Serve immediately with freshly cooked rice and, optionally, with fresh lettuce leaves for wrapping, along with kimchi or other banchan (side dishes).