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.
Two Oven Temperatures. You’ll notice that we start cooking the chicken at a higher temperature and then lower the temp for the remainder of the cooking time. This trick helps to create crispy chicken skin and allows you to slowly roast the chicken too. It’s a win-win. Cooking the whole time at a high temp would give you dried out meat, and cooking only at the lower temp would leave you with soggy chicken skin.
Brine if you can. The difference between a roast chicken that has been brined first and one that hasn’t is subtle, but also very noticeable. Nobody will know if you don’t brine the chicken, but they will definitely comment on how delicious it is if you do.
Use a Thermometer. A simple instant-read meat thermometer is the best way to be certain that you’ve cooked your roast chicken to a safe temperature. If you're not sure how to use it, I wrote a guide on how to use a kitchen thermometer that you might find useful.
Don’t Baste Too Much. Start basting the chicken after it’s been in the oven for 40 minutes or so, but you only need to baste 2-4 times. If you baste more than that you risk making the process take longer, since opening the oven can cause the temperature to drop.
Let it Rest. You will want to slice into this chicken right away because it smells so good, but please be patient. Allowing your roast chicken to rest for 15-20 minutes gives the juices time to redistribute, resulting in perfectly tender and juicy chicken.