Preheat oven to 425°F (220°C). Position a rack in the lower third of the oven. Remove the whole chicken from packaging, discard any giblets, and thoroughly pat the chicken dry inside and out using paper towels. A dry skin is critical for achieving ultimate crackling crispiness.

In a medium mixing bowl, combine softened room-temperature butter, minced garlic, chopped rosemary, thyme, parsley, lemon zest, 1 teaspoon salt, and 1/2 teaspoon black pepper. Mash with a fork until smooth and uniformly combined.

Season the chicken cavity generously with 1/2 teaspoon salt and black pepper. Stuff cavity with halved lemon, halved garlic head, and herb sprigs. Carefully slide your fingers between the chicken breast meat and the skin to loosen the membrane without tearing it. Spread approximately two-thirds of the garlic herb butter directly underneath the skin over the breast meat. Rub the remaining herb butter evenly over the outer skin, legs, and wings.

Tie the drumsticks together with kitchen butcher twine and tuck the wing tips behind the shoulder joints. Place sliced onion wedges and 1 cup chicken broth into the bottom of a heavy roasting pan or cast-iron skillet, then set the chicken on top of the onions breast-side up.

Roast at 425°F (220°C) for 20 minutes to jump-start the crisp skin, then reduce oven temperature to 375°F (190°C). Continue roasting for 50 to 55 minutes, basting once with pan drippings halfway through, until an instant-read digital thermometer inserted into the thickest part of the thigh (avoiding bone) registers 165°F (74°C). Transfer the chicken to a warm carving platter and let rest for 15 full minutes before slicing to allow the succulent juices to redistribute throughout the meat.