Preheat your oven to 375°F and pat the chicken thighs dry with paper towels, then season both sides with salt, pepper, and thyme.
Heat olive oil in a large oven-safe skillet over medium-high heat, then place chicken thighs skin-side down and cook for 5-6 minutes until the skin is golden and crispy.
Flip the chicken and cook for another 3-4 minutes, then remove from the skillet and set aside on a plate.
In the same skillet, add the sliced apples and onions, cooking for about 4 minutes until they start to soften and caramelize slightly.
Add the minced garlic and cook for another minute until fragrant, stirring frequently so it doesn't burn.
Pour in the apple cider and apple cider vinegar, scraping up any browned bits from the bottom of the pan with a wooden spoon.
Add the butter and fresh rosemary, stirring until the butter melts, then nestle the chicken thighs back into the skillet skin-side up.
Transfer the skillet to the preheated oven and bake for 25-30 minutes until the chicken reaches an internal temperature of 165°F.
If you want a thicker sauce, remove the chicken and fruit to a serving platter, then whisk cornstarch with 2 tablespoons of water and stir into the sauce over medium heat until thickened.
Serve the chicken topped with the apples, onions, and drizzled with that gorgeous cider sauce – it's perfect over rice or mashed potatoes!