How to Make It
In a food processor, pulse the basil leaves just until coarsely chopped. Add the olive oil and the coarsely ground black pepper and pulse just until a coarse paste forms. Rub the basil paste all over the lamb and set the roast on a rack set in a roasting pan. Let the roast stand at room temperature for 45 minutes to marinate. After 30 minutes, preheat the oven to 400°.
Meanwhile, in a small heatproof bowl, cover the dried cherries with boiling water and let stand until plump, about 30 minutes. Drain.
Season the lamb with salt and pepper. Roast it on the bottom shelf of the oven for 30 minutes. Reduce the oven temperature to 350° and roast the lamb for 1 1/2 hours longer, or until an instant-read thermometer inserted in the thickest part of the roast reads 125°.
Transfer the roasted lamb to a carving board, cover the roast loosely with foil and let stand for 15 minutes.
Meanwhile, spoon off the fat from the roasting pan and set the pan over 2 burners. Add the red wine and balsamic vinegar and bring to a boil over high heat, scraping up any browned bits from the bottom of the pan with a wooden spoon. Simmer until the liquid is reduced by half, about 5 minutes, then strain the liquid into a small saucepan. Add the beef stock and cook over moderately high heat until reduced to 3/4 cup, about 15 minutes. Add the reconstituted cherries and the butter, season the dried-cherry sauce with salt and pepper and keep warm.
Remove the strings from the lamb roast and cut it into thin slices. Arrange the meat on a platter and surround it with the Caramelized Red Onions. Pour the dried-cherry sauce into a bowl and pass at the table.