Preheat oven to 400 degrees F.
Rub the lamb leg all over with olive oil, and then the pepper and basil. Let the lamb sit at room temperature for at least 20 minutes.
Season the lamb all over with salt, set fat-side up on a rack in a shallow roasting pan, and roast for 30 minutes. Reduce heat to 350 degrees F. and continue to cook for about 1 hour. While the lamb is roasting, peel the onions, cut the ends off, and slice in half into 2 thick rings. Place into a baking pan, drizzle with olive oil, season with salt and freshly ground pepper, and roast alongside the lamb for the last 30 minutes or until they are soft and caramelized. The lamb is done when a thermometer placed in the thickest part of the leg reads 120 to 125 degrees F. Transfer lamb to a cutting board and let rest 10 to 20 minutes.
Spoon off the fat from the roasting pan. Set the pan over 2 burners, add the red wine and balsamic and reduce by half, stirring and scraping up any browned bits from the bottom of the pan. Add the beef broth and reduce until sauce reaches desired consistency. Whisk in the sour cherries and butter. Season with salt and pepper and serve with slices of lamb and red onions.
