Preheat oven to 425°F (220°C). Toss sweet potatoes, apples, rosemary, salt, pepper, and 1 tbsp olive oil; set aside.
Heat remaining olive oil in ovenproof skillet over medium-high. Sear chicken skin-side down until golden, about 2 minutes. Remove from heat.
Add apple cider, honey, and mustard to skillet; simmer until slightly reduced. Whisk in butter, then return chicken, sweet potatoes, and rosemary. Transfer skillet to oven.
Bake until chicken is cooked through and sweet potatoes are tender, about 20 minutes. Rest briefly before serving.
Serve chicken with sweet potatoes and pan drippings.
Notes
Ensure chicken reaches an internal temperature of 165°F (74°C) for safety.