Trim fat from meat. If necessary, cut meat to fit into a 3-1/2- or 4-quart slow cooker. Place meat in cooker. In a medium bowl combine jicama, green onions, vinegar, soy sauce, hoisin sauce, ginger, salt, chili oil, and pepper. Pour over meat.
Cover and cook on low-heat setting for 8 to 10 hours or on high-heat setting for 4 to 5 hours.
If using low-heat setting, turn to high-heat setting. In a small bowl combine cornstarch and the cold water; stir into soy sauce mixture. Cover and cook about 15 minutes more or until thickened.
Spoon meat mixture onto lettuce leaves. Fold bottom edge of each lettuce leaf up and over filling. Fold in opposite sides; secure with a cocktail pick. Serve wraps with the reserved cooking juices and, if desired, cashews.