I highly recommend brining the chicken if the time allows it, see my chicken brine recipe. Take the chicken out of the fridge 30 minutes prior to cooking.
Preheat the oven to 450°F (230°C) standard oven, or 430°F (220°C) convection/fan oven.
Make the butter mixture, in a bowl combine the butter with thyme, parsley, garlic, salt, and pepper. Set aside.
Place the chicken on a cutting board, and pat dry with paper towels. Using kitchen shears, loosen and separate the skin from the chicken, and then use a spoon to push the skin away from the meat without tearing it. Do this to the breast and the thighs and drumsticks, not to the underside of the chicken.
Season the cavity of the chicken with ½ teaspoon of salt.
Use a spoon and spoon most of the garlic butter under the skin and push it from outside to distribute evenly.
Stuff the cavity with the onion halves, and the rosemary sprig. Truss the chicken or just twine the legs with poultry twine.
Transfer to a baking dish, spread the remaining butter on top of the chicken. Drizzle with olive oil and transfer to the oven.
Roast for 15 minutes, then 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 at least twice starting at 30-40 minutes by spooning some pan juices/melted butter 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.