1 (6-pound) leg of lamb, fat trimmed, boned out, rolled and tied by your butcher
1/4 cup extra-virgin olive oil, plus more as needed
1/2 cup freshly ground black pepper, plus more as needed
1 cup chopped fresh basil leaves
Kosher salt
4 medium red onions
1/2 cup red wine
1/4 cup balsamic vinegar
1 cup beef stock
1 cup dried sour cherries, reconstituted in hot water; then drained
1 tablespoon butter