Preheat oven to 450°F. Place pear and onion slices in a single layer in 13 x 9-inch baking dish.

Combine oil, thyme, salt and pepper in a shallow bowl. Spoon half of mixture over pears and onions. Cover baking dish tightly with foil; bake 10 minutes.

Meanwhile, place chicken in remaining oil mixture, turning to coat on all sides.

Uncover pear and onion slices; arrange chicken on top and drizzle with vinegar. Discard remaining oil mixture used to coat chicken. Bake uncovered 20 minutes more or until chicken is done (internal temp 170°F).