Preheat the oven to 425°F (218°C).
Pat the chicken thighs dry with paper towels and place them in a large bowl.
Season with onion powder, garlic powder, smoked paprika, thyme, chopped rosemary, salt, and black pepper.
Toss until the chicken is evenly coated with the seasonings.
Drizzle enough olive oil to lightly coat the bottom of a large oven-safe skillet. Heat over medium to medium-high heat.
Once hot, add the chicken thighs. Sear for approximately 3–4 minutes per side, until nicely golden.
Remove the skillet from the heat. Arrange the sliced red onion and Honeycrisp apples around the chicken.
Add a few fresh rosemary sprigs.
Transfer the skillet to the oven and bake for approximately 20–25 minutes, or until the thickest part of the chicken reaches at least 165°F (74°C), without the thermometer touching the bone.
Transfer the cooked chicken and apples to a clean plate and keep warm.
Carefully place the skillet over medium-low heat.
Add the butter, apple cider, chicken broth, Dijon mustard, and minced garlic.
Whisk gently, scraping up the flavorful browned bits from the bottom of the skillet.
Simmer until the sauce reduces slightly and develops a smooth, glossy consistency.
Taste and adjust the salt and black pepper if necessary.
Return the chicken and apples to the pan.
Spoon the warm sauce generously over the Rosemary Apple Cider Chicken.
Garnish with additional fresh rosemary or thyme and serve warm.