In a small bowl, mix together the softened butter, minced garlic, rosemary, thyme, salt, and black pepper until well combined.
Carefully loosen the skin of the chicken from the breast and thighs using your fingers. Spread half of the garlic butter mixture under the skin and the remaining half all over the outside of the chicken.
Squeeze the juice of one lemon half inside the cavity of the chicken, then place the lemon halves and onion quarters inside the cavity.
Arrange the carrots and potatoes around the chicken in a roasting pan. Drizzle with a little olive oil and season with salt and pepper.
Roast the chicken in the preheated oven for about 1 hour and 15 minutes, or until the internal temperature reaches 165°F (75°C) and the juices run clear.
Let the chicken rest for 10-15 minutes before carving to allow the juices to redistribute.
Notes
For extra crispy skin, broil the chicken for the last 5 minutes of cooking.
Add your favorite vegetables like green beans or Brussels sprouts to the roasting pan for a complete meal.