Start by making the garlic herbs butter. Mix minced garlic, fresh rosemary, and lemon juice into softened butter. Slice onions and lemons for stuffing the body cavity and lining the pan.
Line a cast iron pan with thick slices of red onion. This will keep the bottom of the chicken from getting soggy in the pan drippings. You can use a shallow roasting pan or a lined sheet pan for this as well.
Place the whole chicken on top of the sliced onions breast-side up. Pat the skin dry with a paper towel and make sure the body cavity is empty. Butter the chicken all around and places lemons and rosemary sprigs inside the body cavity.
Tie the legs up with kitchen string. Place the whole bird in a preheat 425° F oven for 20 minutes. Then, turn the heat down to 325°F to finish cooking. Approximately 20 minutes per pound.
Rest the roasted chicken for at least 10 minutes. Carve. First remove legs and thighs by twisting at the joint. Remove the wings the same way. Finally, use your chef's knife to cut the breast away from the breast bone. Slice breast pieces into thick slices.