Prep the Chicken: Lightly poke the chicken thigh with a fork on both sides so the sauce absorbs better.
Combine Sauce: In a small bowl, mix soy sauce, mirin, and sugar together until the sugar dissolves.
Sear the Chicken: Heat vegetable oil in a skillet over medium-high heat. Place the chicken skin-side down and cook for about 4–5 minutes until the skin is crispy and golden brown.
Flip & Cook: Flip the chicken over, cover with a lid, and cook on low heat for about 3–4 minutes until cooked through.
Simmer with Sauce: Wipe off excess oil from the pan with a paper towel. Pour in the sauce mixture and let it simmer over medium heat. Baste the chicken with the sauce until it thickens into a glossy glaze.
Slice & Serve: Slice the chicken thigh into bite-sized pieces. Serve over a warm bowl of rice and top with a drizzle of Japanese mayo if desired!