"In order to get that single flat steak cut from the chicken thigh, it actually takes some precision and some knife handling skills. But all that peeling, deboning, cutting, and marinating is all worth it. I got the savory sweet flavors that bulgogi marinades are known for but made even better with the tender and juicy thighs that were soaked overnight. Serve this with a bunch of Boston/Bibb lettuce, garnished with scallion strips."
Peel skin from chicken thighs and trim off excess fat. Cut each into single flat 'steak,' working around the bone. Save any smaller pieces for cooking as well. Rinse thighs under cold water, removing any film. Place in a large bowl.
Pulse soy sauce, apple, sugar, garlic, sesame oil, gochugaru, and ginger together in a food processor until marinade is smooth.
Pour marinade into the bowl with the chicken and stir to coat. Pierce the chicken with a skewer for extra absorption, if desired. Cover with plastic wrap and refrigerate, 6 to 12 hours.
Heat a skillet over medium heat. Remove chicken from the marinade and add to the skillet. Cook and stir until no longer pink in the center, 15 to 20 minutes. An instant-read thermometer inserted into the center should read at least 165 degrees F (74 degrees C).
Transfer thighs to a serving plate and garnish with sesame seeds.