Remove the neck and giblets from the chicken cavity and discard or reserve for another use. Remove any remaining feathers, rinse the chicken if desired, and pat thoroughly dry with paper towels.
Carefully loosen the skin over the breast by sliding your fingers under the skin and pressing down on the meat to create a pocket for the herb butter.
Preheat the oven to 425°F (220°C). Line a baking sheet with parchment and place a roasting dish on top; spray the dish lightly with nonstick spray. Arrange the sliced onions in the bottom of the dish to form a bed for the chicken.
Season the exterior and cavity of the chicken generously with sea salt and black pepper.
In a bowl, whisk together the lemon juice, olive oil, minced garlic, chopped parsley, chopped rosemary, dried oregano, and softened butter until smooth.
Rub most of the butter-herb mixture all over the outside of the chicken, pressing some under the loosened breast skin. Rub any remaining mixture inside the cavity.
Stuff the cavity with the sliced head of garlic, rosemary sprigs, and the quartered lemons. Tie the legs with kitchen twine, tuck the wing tips under the body, and place the chicken breast-side up on the onion bed in the roasting dish.
Roast the chicken in the preheated oven for about 1 hour 15 minutes, basting at 30 minutes and then roughly every 20 minutes. If the chicken browns too quickly, tent loosely with foil.
Roast until the juices run clear when a knife is inserted between the leg and body or until a meat thermometer inserted into the thickest part of the thigh reads 165°F (74°C).
Optional: baste once more, then broil for 2 to 3 minutes to crisp and deepen the color of the skin. Watch carefully to avoid burning.
Remove the chicken from the oven, tent loosely with foil, and let rest for about 15 minutes before carving. Serve with pan juices drizzled over slices.