Wrap the steak in plastic wrap and freeze for 20 minutes or so to firm up. Thinly slice across the grain into about 1/8″ slices.

Place the beef and sliced onion in a gallon freezer bag or a large bowl and set aside.

In a food processor, combine the chunked onion, garlic, Asian pear, and grated ginger and process until it’s broken down. Add the soy sauce, gochuchang (if using), vinegar, sesame oil, and sugar and pulse until combined. Pour over steak and onions and mix to combine. Add the sliced green onion and pepper and mix again. Refrigerate for 30 minutes up to overnight.

When ready to cook, preheat a cast-iron pan or large skillet over high heat, work in batches, or use two skillets. Add the meat, onions, and marinade to the pan. Cook, stirring constantly until the meat is cooked through, two to three minutes.

Push the meat off to one side and tilt skillet so the juices flow over to the other. Nudge the skillet so the majority of the heat from the eye is over the side with most of the juices and the meat is just off the heat. Continue to cook until the juices begin to brown and caramelize. Push the cooked meat back into the now caramelized juices and continue to cook a minute or two until the meat is coated and seared with the juices, two to three more minutes.

Repeat with the rest of the ingredients.

Garnish with sliced green onion and sesame seeds and serve with desired accompaniments.

Note: if you wish to serve over rice, you might not want the beef/sauce caramelized as much as those juices are wonderful as they mingle with rice.