1tablespooncold butteroptional (omit for dairy-free)
Remove the lamb from the refrigerator and let sit at room temperature for 15 minutes. Preheat the oven to 450ºF.
Season the lamb on all sides generously with salt and pepper. Put the oil in a large skillet and heat over medium-high heat. Sear the lamb racks (one at a time) on all sides, including the ends, until browned, about 4 minutes per side. Place on a baking sheet and roast for 15 minutes or until an instant read thermometer reaches 145ºF for medium-rare. Remove from the oven, cover with foil, and let sit for 10 minutes.
In a large skillet, combine the cherries, broth, and thyme. Bring to a boil over medium-high heat. With a potato masher or the back of a spoon, mash the cherries. Let cook until the liquid has reduced by half, 10-15 minutes. Take the sauce off the heat and remove the thyme sprigs and discard. Stir in the balsamic vinegar and butter, if using, stirring until the butter is melted. Season to taste with salt and pepper.
Cut the lamb rack between the bones and serve with the sauce.