Set the oven rack in the center of the oven. Place a 12-inch cast iron skillet on the rack, then preheat the oven to 500ºF.

While the skillet is heating, pat the chicken thighs dry with paper towels, then season with salt and pepper.

When the oven reaches 500ºF, using pot holders, remove the skillet from the oven and place it on the stove over medium heat. (You can turn off the oven at this point.) Add the vegetable oil, and heat until it is just smoking. Place the chicken thighs in the skillet, skin side down. Place a weighted Dutch oven over the chicken to weight it down*, and cook until the skin is crispy and browned, about 16 to 20 minutes. Start checking the chicken at 10 minutes and adjust the heat as it is needed.

When the chicken is crispy and browned, remove the Dutch oven and flip the chicken. Cook the second side (without being weighted down) until browned and crispy and when the chicken reaches 175ºF, 2-4 minutes. Transfer the chicken to a plate.

In a bowl, whisk the soy sauce, sugar, mirin, garlic, ginger and cornstarch together. Pour the fat off of the cast iron skillet and add the mixture. Bring to a simmer over medium heat. Cook the mixture, stirring occasionally, until it thickens and becomes glossy, about 2 minutes. Add the chicken and any accumulated juices back to the skillet with the sauce. Turn the chicken to coat in the sauce. Sprinkle with the green onions.

Serve the chicken with the pan sauce.