Preheat your oven to 400°F (200°C).

In a small saucepan, combine the apple cider (or apple juice), Dijon mustard, and rosemary sprig. Bring to a boil over medium heat, then reduce the heat to low and let it simmer for about 10-15 minutes or until the liquid is reduced by half.

While the cider mixture is reducing, season the chicken thighs with kosher salt and black pepper on both sides.

Heat the vegetable oil in a large oven-safe skillet over medium-high heat. Once hot, add the chicken thighs skin-side down.

Sear the chicken for about 5-7 minutes or until the skin is golden brown and crispy. Flip the chicken thighs over and cook for an additional 3 minutes.

Remove the skillet from the heat. Discard the rosemary sprig from the cider mixture and pour the reduced apple cider mixture over the chicken thighs.

Add the unsalted butter to the skillet, placing it around the chicken.

Transfer the skillet to the preheated oven and bake for about 20-25 minutes, or until the chicken is cooked through and reaches an internal temperature of 165°F (74°C).

Remove the chicken from the oven and let it rest for a few minutes before serving. Spoon some of the glaze from the pan over the chicken before serving.