Preheat the oven to 425 degrees F. In a medium sized bowl add the butter. Mix in olive oil, rosemary, garlic, and lemon zest. Mix well to combine.
Season the chicken generously all over with salt and pepper, including the inside of the cavity. Brush the garlic and butter mixture all over the chicken.
Squeeze the juice of 2 lemons over the chicken. Stuff the chicken with lemon slices, onion, and rosemary. Place the chicken into a roasting pan.
Place into the oven and bake for 45 minutes, then baste. Return to the oven and cook for another 15 minutes or until golden and fully cooked (the internal temperature has reached 165 degrees F. If the chicken starts to brown too quickly you can tent it with foil. Let the chicken cool for 15 minutes before carving.