Remove the neck and giblets from the chicken cavity and remove any leftover feathers. Rinse the chicken and pat very dry with paper towels. Carefully loosen the skin over the breasts by sliding your fingers between the skin and meat.
Preheat the oven to 425°F (218°C). Line a baking sheet with parchment and place a roasting pan or ovenproof baking dish on top to catch any drips. Spray or lightly oil the dish.
Arrange the sliced onions in a single layer on the bottom of the roasting dish and set the chicken on top, breast side up. Season the entire chicken liberally with sea salt and black pepper.
In a mixing bowl, combine 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 over the outside of the chicken, spreading some under the loosened skin over the breasts and rubbing the remainder inside the cavity.
Stuff the cavity with the quartered lemons, the halved/sliced head of garlic, and the 3 rosemary sprigs. Tie the legs together with kitchen string and tuck the wing tips under the body.
Place the dish in the preheated oven and roast for about 1 hour 15 minutes total, basting once at 30 minutes and then roughly every 20 minutes. If the chicken is browning too quickly, tent loosely with foil.
Cook until the juices run clear when you pierce between the leg and body and the internal temperature reaches 165°F (74°C) on a meat thermometer.
For extra-crispy skin, baste once more and broil for 2–3 minutes, watching closely to avoid burning.
Remove the chicken from the oven, tent with foil, and let rest for about 15 minutes before carving. Spoon pan juices over slices when serving.
