Step 1: Prepare and Season the Chicken
Pat the chicken thighs completely dry with paper towels. This is the most important step for achieving crispy skin. In a small bowl, mix the kosher salt, black pepper, garlic powder, and smoked paprika. Sprinkle the seasoning mix evenly over both sides of the chicken thighs.

Step 2: Sear to Golden Perfection
Heat a large cast-iron or heavy-bottomed oven-safe skillet over medium-high heat. Add the tablespoon of oil. Once the oil is shimmering, place the chicken thighs in the skillet, skin-side down. Sear without moving them for 6-8 minutes, until the skin is deeply golden brown and crispy. Flip and cook for another 3-4 minutes on the other side. The chicken will not be cooked through at this point. Remove the chicken from the skillet and set it aside on a plate.

Step 3: Sauté the Aromatics
Reduce the heat to medium. If there is an excessive amount of rendered chicken fat, pour off all but about 1 tablespoon. Add the minced shallot to the skillet and cook for 2-3 minutes, until softened and fragrant. Add the minced garlic and cook for another 30 seconds until fragrant. (If using fresh apple slices, add them now and cook for 2-3 minutes until just beginning to soften).

Step 4: Deglaze and Build the Sauce
Pour in the apple cider, using a wooden spoon to scrape all the browned bits from the bottom of the pan. This is where the deep flavor comes from. Add the fresh thyme sprigs, whole-grain mustard, and Dijon mustard. Whisk to combine. Bring the sauce to a simmer.

Step 5: Simmer and Reduce
Return the seared chicken thighs to the skillet, skin-side up, nestling them into the sauce. The sauce should come about halfway up the sides of the chicken. Reduce the heat to maintain a gentle simmer. Cover the skillet and let it cook for 15-20 minutes, or until the chicken reaches an internal temperature of 165°F (74°C) at the thickest part.

Step 6: Reduce the Sauce
Once the chicken is cooked, transfer it back to the plate. If you used thyme sprigs, fish them out and discard them. Increase the heat to medium-high and let the sauce boil vigorously for 3-5 minutes, until it reduces by about a third and thickens enough to coat the back of a spoon. (If using dried cranberries, add them now to plump up).

Step 7: Finish and Glaze
Reduce the heat to low. Stir in the apple cider vinegar. Remove the skillet from the heat entirely. Whisk in the cold butter, one piece at a time, until the sauce becomes glossy and slightly thickened. This process, called monter au beurre, gives the sauce a beautiful sheen and velvety texture. Taste the sauce and adjust seasoning with salt and a pinch of white pepper. Return the chicken to the skillet, spooning the glossy sauce over the top to glaze it.