In a large bowl, stir together the soy sauce, mirin, brown sugar, grated pear, garlic, ginger, sesame oil and black pepper until the sugar dissolves.
Add the chicken pieces and the onion wedges and mix with your hands so every piece is coated. Cover and marinate at room temperature for 30 minutes, or in the fridge for up to 24 hours. If it has been in the fridge, let it sit out for 15 minutes before cooking so it sears rather than steams.
Lift the chicken and onion out of the marinade with tongs or a slotted spoon, letting the excess drip back into the bowl. Keep the leftover marinade. Wet chicken will not char.
Heat a 12 inch cast iron or heavy stainless skillet over high heat until a drop of water skitters and evaporates instantly. Add the neutral oil, then lay in half the chicken and onion in a single layer. Leave it alone for 2 to 3 minutes until the underside is deeply browned with charred edges, then flip and cook another 2 minutes. Transfer to a plate and repeat with the second batch.
Return all the chicken to the pan. Pour in the reserved marinade and add the scallion lengths. Boil hard for 1 to 2 minutes, tossing, until the liquid reduces to a sticky glaze that clings to the chicken and the thickest piece reads 165F (74C) inside.
Turn off the heat, scatter over the sesame seeds and sliced scallions, and serve with rice, kimchi and lettuce leaves for wrapping if you have them.