1. Preheat Oven and Prepare Baking Dish:
* Preheat your oven to 400°F (200°C).
* Lightly grease a 9×13 inch baking dish or a similar-sized oven-safe skillet with a little olive oil or cooking spray.

2. Prepare the Chicken Thighs:
* Pat the chicken thighs thoroughly dry with paper towels. This is a crucial step as it helps the sauce adhere better and allows the chicken to brown nicely rather than steam.
* Season the chicken thighs on both sides with about ½ teaspoon of the salt and ¼ teaspoon of the black pepper. (The rest of the salt and pepper will go into the sauce).
* Arrange the seasoned chicken thighs in a single layer in the prepared baking dish. Try not to overcrowd the dish; if necessary, use two smaller dishes.

3. Whisk Together the Honey Mustard Sauce:
* In a medium-sized bowl, combine the honey, Dijon mustard, whole grain mustard, 1 tablespoon of olive oil, apple cider vinegar (or lemon juice), minced garlic (or garlic powder), onion powder, smoked paprika (or sweet paprika), the remaining ¼ teaspoon of salt, and the remaining ¼ teaspoon of black pepper.
* Whisk thoroughly until all the ingredients are well combined and the sauce is smooth and emulsified. Taste the sauce and adjust seasoning if needed (e.g., a bit more honey for sweetness, a touch more vinegar for tang, or a pinch more salt).

4. Coat the Chicken with Sauce:
* Pour about two-thirds of the honey mustard sauce evenly over the chicken thighs in the baking dish. Use a brush or the back of a spoon to ensure each thigh is well-coated on the top and sides.
* Reserve the remaining one-third of the sauce for basting later. This ensures a fresh, glossy glaze at the end.

5. (Optional) Marinate:
* For even deeper flavor, you can cover the baking dish and let the chicken marinate in the refrigerator for at least 30 minutes, or up to 4 hours. If you’re short on time, you can skip this step, and the chicken will still be delicious. If marinating, allow the chicken to sit at room temperature for about 15-20 minutes before baking.

6. Bake the Chicken:
* Place the baking dish in the preheated oven.
* Bake for 20 minutes.

7. Baste and Continue Baking:
* After 20 minutes, carefully remove the baking dish from the oven.
* Spoon or brush the reserved one-third of the honey mustard sauce over the chicken thighs. You can also spoon some of the pan juices over the chicken.
* Return the baking dish to the oven and continue to bake for another 10-20 minutes. The total baking time will be approximately 30-40 minutes for boneless, skinless thighs, or 40-50 minutes for bone-in, skin-on thighs.
* The chicken is cooked through when it reaches an internal temperature of 165°F (74°C) in the thickest part when checked with a meat thermometer. The juices should run clear when pierced with a fork, and the surface should be nicely caramelized and glazed.

8. (Optional) Broil for Extra Caramelization:
* If you want a more deeply caramelized and slightly crispier top, you can switch the oven to the broil setting (High) for the last 2-3 minutes of cooking.
* IMPORTANT: Watch the chicken very carefully under the broiler, as the honey in the sauce can burn quickly. Keep the oven door slightly ajar and do not walk away. Remove as soon as it reaches your desired level of browning.

9. Rest and Serve:
* Once cooked, remove the baking dish from the oven.
* Let the chicken rest in the baking dish for 5-10 minutes before serving. This allows the juices to redistribute, resulting in more tender and flavorful chicken.
* Spoon some of the delicious pan sauce over the chicken thighs.
* Garnish with freshly chopped parsley or chives, if desired.