1 pound Brussels sprouts, trimmed and halved or quartered if large
2 sweet apples (such as Gala), cut into wedges
1 red onion, cut into wedges (root left intact)
2 small sprigs rosemary, plus 1 teaspoon finely chopped leaves
2 tablespoons extra-virgin olive oil
Kosher salt and freshly ground pepper
2 tablespoons unsalted butter
4 skinless, boneless chicken breasts (about 8 ounces each)
2/3 cup apple cider
1 teaspoon apple cider vinegar
• 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.