Pat the chicken dry with paper towels.
Place the chicken in a roasting pan and season with salt and pepper from all sides. Pour the herb butter all over the chicken, and with a brush spread it evenly all over the chicken.
Stuff the cavity with the quartered onion, rosemary sprigs, and lemon wedges.
Truss the chicken legs with kitchen twine, season with salt and pepper again, and transfer to the oven.
Roast for 15 minutes at 450°F (230°C) standard oven, or 430°F (220°C) convection/fan oven.
Lower the heat to 400°F (200°C) standard oven, or 380°F (190°C) convection/fan oven. And roast for 45 minutes (this changes depending on the size of your chicken). Baste twice starting at 30-40 minutes by spooning some pan juices over the chicken.
Check the internal temperature of the chicken, when it reaches 165°F (74°C) at the thickest part of the thigh, the chicken is ready. Remove from the oven.
Allow the chicken to rest for 15-20 minutes uncovered before carving and serving.