Wrap steak in plastic wrap, and place in the freezer for 30 minutes. Unwrap and slice across the grain into 1/4-inch thick slices.
In a medium bowl, combine pear, soy sauce, brown sugar, sesame oil, garlic, ginger and gochujang. In a gallon size Ziploc bag, combine soy sauce mixture and steak; marinate for at least 2 hours to overnight, turning the bag occasionally.
Heat 1 tablespoon vegetable oil in a cast iron grill pan over medium-high heat. Working in batches, add steak to the grill pan in a single layer and cook, flipping once, until charred and cooked through, about 2-3 minutes per side. Repeat with remaining 1 tablespoon vegetable oil and steak.
Serve immediately, garnished with green onions and sesame seeds, if desired.
Notes
Freeze the steak before slicing. The key to bulgogi is using paper thin slices of meat. Throwing the steak in the freezer for 20-30 minutes will make it easier to slice the meat into very thin, even pieces. Pre-sliced bulgogi beef is also readily available at Korean and Japanese grocery stores.
Slice against the grain. Always slice the beef perpendicular to the muscle fibers, shortening the fibers to allow for a more tender bite.
Pear adds maximum flavor. Adding coarsely grated pear will not only add a subtle sweetness to the marinade, but it will also help tenderize the meat. Korean pear or Bosc pear can be used.
Use reduced sodium soy sauce. Reduced sodium has less sodium and less salt without compromising flavor.
Watch the marinating time. Marinate the beef for at least 2 hours but no longer than 24 hours as the natural enzymes of the pear will break down the protein too much, resulting in mushy, mealy beef.
Use a hot cast iron grill pan. A heavy bottom pan, such as a cast iron grill pan, retains heat much better than a nonstick skillet, allowing for even cooking, a better sear and smoky, caramelized flavors.
Cook in batches. Work in batches to properly sear the meat for maximum caramelization and smoky flavors. An overcrowded pan will unfortunately steam the beef instead.
Freeze as needed. Marinated bulgogi is very freezer-friendly, great for meal prep and making weeknight meals even easier and quicker.