Mix garlic, salt and pepper; rub over beef. Transfer to a 4-qt. slow cooker. Pour coffee around meat. Cook, covered, on low until meat is tender, 8-10 hours.
Remove roast to a serving plate; keep warm. Transfer cooking juices to a small saucepan; skim off fat. Bring to a boil. In a small bowl, mix cornstarch and water until smooth; gradually stir into cooking juices. Bring to a boil; cook and stir until thickened, 1-2 minutes. Serve with roast.