Put the beef in the freezer for 20 to 30 minutes so it firms up, then slice it 3mm thick across the grain. Thin slices matter more than anything else in this recipe.
In a large bowl, mix the soy sauce, grated pear, brown sugar, sesame oil, garlic, ginger and black pepper. Add the beef and the sliced onion and mix with your hands until every slice is coated. Cover and marinate 30 minutes at room temperature, or up to 24 hours in the fridge.
Lift the beef and onion out of the bowl and let the excess marinade drain off. Wet beef steams instead of searing. Discard the leftover marinade.
Heat a large cast iron skillet or wok over high heat until it just starts to smoke. Add the neutral oil, then a third of the beef and onion in a single layer. Leave it alone for 60 to 90 seconds until the underside browns and the sugar starts to caramelize, then toss and cook 1 minute more. Transfer to a plate.
Repeat with the remaining beef in two more batches, letting the pan get hot again between each one. Add the scallion pieces with the last batch.
Return everything to the pan for 30 seconds to warm through and pick up any caramelized bits stuck to the bottom. Turn off the heat.
Divide the hot rice between four bowls. Top with the beef and onion, then the sliced scallion and sesame seeds. Serve with kimchi on the side if you have it.