Heat coals or gas grill for direct heat. Cut four 18x12-inch pieces of heavy-duty aluminum foil. Spray with cooking spray. Place one-fourth of the sweet potato slices on one side of each foil piece. Sprinkle with salt. Cut butter into small pieces; sprinkle over sweet potato. Top with pork chops and peaches. Sprinkle with cinnamon.
Fold foil over pork mixture so edges meet. Seal edges, making tight 1/2-inch fold; fold again. Allow space on sides for circulation and expansion.
Cover and grill packets 4 to 5 inches from medium heat 20 to 22 minutes or until sweet potatoes and peaches are tender. Place packets on plates. Cut large X across top of each packet; fold back foil. Sprinkle with onions.