Preheat oven & prep pan: Preheat oven to 425°F. Place a few celery halves in a cast iron skillet, as a makshift wrack for the chicken to rest on.
Clean and dry chicken: Make sure the cavity is empty and remove giblets/neck. Pat the chicken completely dry with paper towels, removing any excess skin or feathers.
Season the chicken: Place chicken breast-side up on the rack. Season generously with salt and pepper all over — including the underside and inside the cavity.
Make garlic butter rub: In a small bowl, mix ¼ cup melted butter, 1 tablespoon avocado oil, minced garlic, juice of 1 lemon, and rosemary. Rub the mixture all over the chicken, including under the skin.
Stuff the cavity: Fill the inside of the chicken with halved lemon, onion quarters, carrots, and halved garlic head. Arrange any extras around the chicken. Tie the legs with kitchen twine (Optional).
Roast the chicken: Place the chicken in the oven uncovered and roast at 425°F for 30 minutes.
Lower temp and baste: Reduce heat to 350°F, baste with the remaining ¼ cup melted butter, and continue roasting for 1 hour or until internal temp reaches 165°F. I usually cover the chicken towards the end to keep it from browning too much.
Rest and serve: Baste again with pan juices and let rest 15–20 minutes before slicing. Serve warm with pan juices and your favorite sides.