Position racks in the upper and lower thirds of the oven; preheat to 450 degrees F. Toss the Brussels sprouts, apples, red onion and rosemary sprigs with the olive oil, 1/2 teaspoon salt and a few grinds of pepper on a baking sheet. Spread in a single layer and roast on the upper oven rack, flipping halfway through, until tender and browned, 25 to 30 minutes.
Meanwhile, heat 1 tablespoon butter in a large ovenproof skillet over medium-high heat. Season the chicken on both sides with salt, pepper and the chopped rosemary. Add the chicken to the skillet and cook until browned on the bottom, about 6 minutes. Flip and cook 2 more minutes, then pour the cider into the skillet around the chicken. Transfer to the lower oven rack and roast until just cooked through, 10 to 12 minutes.
Transfer the chicken to a cutting board to rest and return the skillet to medium-high heat. Simmer until the pan sauce is reduced by half, about 1 minute. Remove from the heat and swirl in the remaining 1 tablespoon butter and the vinegar; season with salt and pepper.
Slice the chicken and divide among plates along with the roasted vegetables and apple. Spoon the sauce over the chicken.
Photograph by Ryan Dausch