Preheat the oven to 425°F (220°C).
Pat the chicken thighs dry with paper towels. Season them with onion powder, garlic powder, smoked paprika, fresh thyme, chopped rosemary, salt, and black pepper.
Heat a drizzle of olive oil in a large oven-safe skillet over medium heat.
Once the pan is hot, add the chicken thighs skin-side down if using skin-on thighs. Sear for about 3–4 minutes per side, or until nicely golden.
Remove the skillet from the heat. Arrange the sliced red onion and Honeycrisp apples around the chicken. Add the rosemary sprigs.
Transfer the skillet to the oven and bake for about 20–25 minutes, or until the chicken reaches an internal temperature of 165°F (74°C) at the thickest part.
Carefully remove the chicken and apples from the skillet and set them aside.
Place the skillet over medium-low heat. Add the butter, apple cider, chicken broth, Dijon mustard, and diced garlic.
Whisk the sauce periodically as it gently simmers and cooks down until slightly thickened.
Return the chicken and apples to the skillet. Spoon the warm apple cider sauce over the chicken and garnish with additional rosemary. Serve immediately.