Slice the chicken: if the thighs feel very soft, firm them in the freezer for 20 to 30 minutes first. Slice across the grain into pieces about 1/4 inch thick and 2 inches long.
Mix the marinade: in a large bowl, stir the soy sauce, grated pear, brown sugar or honey, sesame oil, garlic, ginger, black pepper, and scallion whites until the sugar dissolves.
Marinate: add the chicken and toss until every piece is coated. Cover and refrigerate for at least 30 minutes and up to overnight.
Drain, but do not dry: lift the chicken from the marinade with tongs and let the excess drip back into the bowl. Reserve the leftover marinade if you want a quick pan sauce.
Sear in batches: heat a large cast-iron skillet, heavy stainless pan, or wok over high heat until very hot. Add the neutral oil and swirl. Lay in half the chicken in a single layer and leave it alone for 2 minutes. Flip and cook 2 to 3 minutes more until the edges look caramelized and the chicken is cooked through. Move to a plate and repeat with the second batch.
Glaze and finish: return all the chicken to the pan. If you reserved marinade, add 2 to 3 tablespoons of it and let it bubble for 30 to 60 seconds until glossy. Toss in the scallion greens, take the pan off the heat, and sprinkle with sesame seeds. Serve at once over rice.