Procedure
The night before: Thoroughly rub the chicken dry, inside and out, with paper towels. You want it to be as dry as possible to produce crispy skin. Season generously with salt (but keep in mind you are not trying to salt-cure the thing; season as you would if you were cooking something right away). Place uncovered in the fridge overnight.
The next day, wipe off any excess salt or extracted moisture. With the chicken placed breasts-up, insert a small spatula or the back of a spoon from the bottom to carefully separate the skin from the meat. Remove the endcaps off the drumsticks, if desired. Then, on each drumstick, make the following 3 deep slits: in the thickest part of the drumstick, in between the leg and thigh, and in the thickest part of the underside of the thigh. (This is what will help cook the bird evenly.)
Place a shallow, large cast iron brasier or skillet on the middle rack of your oven, and preheat to 475°F. (Your vessel should be much larger than the chicken, to accomadate the bread and allow for airflow.)
Roughly chop a mixture of fresh herbs. Drizzle a bit of olive oil on top and season with a couple pinches of salt. Mix together with your fingers then stuff the herb mixture under the skin of the chicken, in both the breast area and through the slits you made in the drumsticks. Rub the residue from your hands onto the outside of the chicken. Stuff 1 tablespoon of butter under the skin over each breast. Tuck the wing tips under the torso of the chicken.
Cut the head of garlic in half and stuff into the cavity. Stab the lemon all over with a paring knife, then microwave for 30 seconds. Stuff into the cavity. Cut the onion (skin on) in half and set aside.
When the oven is ready, carefully add 1 tablespoon of wagyu fat or butter into the preheated cast iron pan and coat the bottom of the pan. Add the onion halves then place the chicken, breast-side-down, in the pan. Bake for 30 minutes.
After 30 minutes, flip the chicken over and tie the ends of the drumsticks together with kitchen twine. Return to the oven for another 15 minutes.
In the meantime, tear up the bread into about 1-inch chunks. Add the bread evenly around the chicken, then bake for another 15 minutes (or until juices run out clear when you stab the thigh with a toothpick).
Remove the chicken from the pan and place on a cutting board to rest. Cut off the twine and remove the lemon and garlic. Allow the chicken to rest for at least 15 minutes before carving. See notes above for what to do with the bread…or simply trust your instincts ;)