Heat olive oil in a large oven-safe skillet over medium-high heat. Season chicken thighs with salt and pepper, then sear in the skillet for 5-7 minutes until golden brown.

Remove the chicken and set aside. In the same skillet, add butter and sauté chopped red onion for about 3 minutes until softened. Add sliced Honeycrisp apples and rosemary, cooking for another 4-5 minutes.

Pour in apple cider and chicken broth (or white wine), stirring to combine. Bring to a simmer and whisk in Dijon mustard. Thicken the sauce for approximately 5 minutes until syrupy.

Return chicken to the skillet, skin-side up. Preheat oven to 375°F (190°C) and roast for 20-25 minutes until internal temperature reaches 165°F (74°C).

After roasting, whisk in minced garlic to the sauce, simmer for an additional 2-3 minutes, then drizzle the sauce over the chicken before serving.