Combine the minced garlic, lemon zest, ½ tsp of kosher salt, and black pepper in a small bowl. Add enough oil to make a paste. Set aside.
Place the chicken on a board with the breast side down.
Find the bone that runs down the middle. Cut it away from the meat on one side and then cut it on the other side to remove the bone.
Find the bone that connects the two sides of the breast. Using a pairing knife, remove it. It's ok if you can't remove it, the chicken will still be flatten, just not as flat as possible.
Turn the chicken over and flatten it down with your hands. Gently pull up the skin to place the seasoning mixture underneath the skin. Then wash those hands!
Grab enough onions, celery, and/or carrots to place underneath the chicken. Quarter the onions and break the celery and carrots in half. Place on a roasting pan. Then put the chicken (breast side down) on top of the vegetables
Turn the broiler on or set your oven to 500-550 degrees or as high as it will go. Place the pan into the oven. Drizzle with oil and sprinkle ½ tsp of salt on the skin (both sides).
Broil for 10 minutes then flip over, so the breast side is on top.
Broil for 20-25 minutes or until the chicken's internal temperature is 160 when stuck into a thigh. Check the chicken after 15 minutes, if it's getting too brown you can carefully cover it with foil and put it back in until the internal temperature is reached.
Allow the chicken to rest for 10 minutes before carving.
Once you remove the chicken from the pan, you can carefully drain any of the liquid. This can be use to dip the chicken in - it's delicious.