Adapted from Smitten Kitchen
Preheat oven to 450 degrees with the rack in the middle. Pat chicken dry and season with kosher salt and pepper. Heat oil in a cast iron skillet or one that's ovenproof over medium high until it shimmers. Working in batches to avoid steaming, brown chicken, skin side down first (if using). Let the chicken sit on the first side at least 2.5-3 minutes to get a nice sear. Do not move it around or it won't brown nicely! Turn again and brown for another 2-2.5 minutes.
Return all the pieces to the pan, skin side up (if using) and surround with grapes, OPTIONAL olives, and shallots. Roast the chicken in the oven until juices run clear - about 15-20 minutes. Transfer chicken, grapes, and optional olives to a platter and then add your chicken or vegetable broth to the skillet. Bring it to a boil, scraping up browned bits, until the liquid has reduced by half (3 minutes). Pour sauce over the chicken and top with rosemary.