Preheat the oven to medium-high heat, about 350˚F to 400˚F.
Pat chicken dry with paper towels.
Place chicken breast-side down, with the neck facing you on the cutting board.
Use high-quality shears to cut along one side of the spine, separating it from the ribs. Be sure to cut as close to the spine as possible. Repeat this on the other side of the spine, removing it from the chicken. Discard the spine, or reserve to use in chicken stock.
Flip the chicken over so the breast is facing up. Use the palms of your hands to press down along the breastbone. This will flatten the bird out more.
Sprinkle half of the Head Country Original Championship Seasoning on top. Rub it all over the skin, making sure to get it in all the nooks and crannies.
Brush the grill grates with oil. Place the chicken skin side down over direct heat and sprinkle with the other half of the Head Country Original Championship Seasoning. Cook until the chicken skin starts to get a little crispy, about 5 minutes. Reduce the heat to medium, about 300˚F to 350˚F. Flip the bird over and brush the skin with BBQ sauce in an even layer. Cook until the thickest part of the chicken registers 165˚F on an internal meat thermometer. Brush the chicken with sauce every 20 minutes or so. It will take about 45 minutes to cook through.
Remove from the grill and let rest for 10 minutes before carving. Sprinkle with sliced green onion.