Slice the beef paper-thin against the grain. If it's not pre-sliced, freeze it for 45 minutes first so it firms up enough to cut thinly and evenly.
Grate the Asian pear and half the onion straight into a mixing bowl, so you catch all the juice.
Whisk in the soy sauce, sugar, mirin, garlic, ginger, sesame oil, and black pepper until the sugar dissolves.
Add the beef, toss to coat every slice, and press it down so the marinade covers the surface.
Cover and refrigerate for at least 30 minutes; 2 to 4 hours is better, and overnight is the upper limit before the texture turns mushy.
Heat a wide skillet over high heat with the neutral oil until it's just starting to shimmer.
Lift the beef out of the marinade letting excess liquid drip off, and lay it in a single layer without crowding the pan; cook in batches if needed.
Sear undisturbed for about 90 seconds until the edges char and curl, then flip and cook 60 to 90 seconds more.
Add the sliced onion, carrot, and mushrooms if using, and toss everything for 1 to 2 minutes until the vegetables just soften.
Scatter the scallions in during the last 30 seconds, then serve immediately over rice.
