Use hickory wood charcoal in your grill and let your grill get to 350 degrees.
Peel the eyes of the potatoes keeping most of the skins on, make the potatoes all uniform in the size put in an oven proof skillet. Slice the onion in 1/2 inch strips place in a skillet with potatoes and toss with oil and kosher salt
Lay the chicken breast-side down. Use kitchen shears to cut out the backbone. Open the chicken like a book to reveal the ribs and sternum. Use your hands and body weight to crack the sternum and flatten the breasts and rib cage.
Spread seasoned ghee all over the chicken top and bottom.
Put the potatoes on the bottom rack of the grill and place the chicken on the top rack, skin side up, directly over the potatoes. Close the lid and cook for 1 hour and 15 minutes or until the temperature of the chicken is 160 to 165. You do not have to flip the chicken. let set for 15 minutes before cutting. Make sure to pour the juice from the skillet over the potatoes and chicken.
If you do not have a double decker grill take four bricks and lay them two on top of each other and lay a cookie sheet on the brick over the potatoes and lay your chicken on top of the cookie sheet.