Combine the marinade ingredients in a glass measuring cup(everything except the chicken). Pulse with an immersion blender a few times until somewhat smooth.
Place the chicken, breast side down, on a cutting board. Using sturdy kitchen shears or a very sharp knife, cut from the neck to the tail end, along both sides of the backbone, to remove it. Take special care if you are using a knife; you’ll be cutting through small bones and will have to use some force.
Once the backbone is out, you’ll be able to see the interior of the chicken. Make a small slit in the cartilage at the bottom end of the breastbone. Then, placing both hands on the rib cage, crack the chicken open like a book. Run your fingers along either side of the cartilage in between the breasts to loosen it from the flesh. Grab the bone and pull up on it to remove it along with the attached cartilage. The chicken should now lay flat.
Place the chicken inside a 13 X 9 shallow baking pan and carefully rub marinade under the skin and then turn to coat it evenly all over with the marinade. Refrigerate for about 2 hours.
Prepare the grill for direct cooking over medium-low heat.
Brush the cooking grates clean. Place the chicken, bone side down, over direct medium-low heat and put your cast-iron skillet, or weight a sheet pan down with 2 foil-wrapped bricks, directly on top. Close the lid and cook for 20 to 30 minutes. This WILL smoke alot -- don't panic. Remove the skillet, turn the chicken over, replace the weight, close the lid, and cook until the juices run clear and an instant-read thermometer inserted into the thigh (not touching the bone) registers 170°F, 20 to 30 minutes. Remove from the grill and let rest for 3 to 5 minutes. Serve warm.