Grate the pears on the large holes of a box grater into a large bowl. Make sure to collect the juices along with the peel and flesh of the pear. Grate ginger and garlic on the small holes of the grater into the same bowl.
In the same bowl, add the white parts of the green onions, soy sauce, brown sugar, sesame oil, and red pepper flakes. Stir to combine.
Slice the steak into thin pieces across the grain of the meat. Add to the marinade and toss to coat.
Cover the bowl and let the beef marinate for 30 minutes to 8 hours.
When ready to cook, heat a large, wide cast iron or other pan over medium-high heat. Add a tablespoon of the oil to the hot pan and about a pound of meat. Sear until slightly charred and cooked through, 1 to 2 minutes on each side. Transfer to a serving dish. If you are using more than a pound of meat, repeat with another tablespoon of oil and the rest of the meat.
Garnish with the green parts of the green onion. Serve over rice (optional).