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