In a large bowl, whisk together soy sauce, sugar, sesame oil, Asian pear, garlic, and ginger. When sugar has dissolved, add beef, scallion, and onion slices to bowl and toss to coat. Cover and refrigerate for at least 1 hour or up to overnight.
In a large cast iron or heavy stainless steel skillet, heat 1 tablespoon (15ml) oil over medium-high heat until shimmering. Working in batches, add beef and onions, working quickly to arrange them in a flat, even layer against bottom of skillet; try not to overcrowd pan. Cook on one side for 1 minute, then flip and cook second side until browned and crisp in spots, about 2 minutes longer.
Transfer cooked beef and onions to a plate and repeat with remaining beef, onions, and oil.