Preheat oven to 475 degrees F.
Season chicken with salt and pepper. Remove the neck and giblets and transfer the chicken to a baking sheet lined with foil (or some other easy-to-clean workspace). Pat chicken dry with paper towels and liberally sprinkle all sides with salt and pepper.
Prepare garlic herb butter - Melt 4 tablespoons of butter in a small bowl. Add the olive oil, juice from one lemon, 3 tablespoons fresh minced herbs, and 5 cloves of minced garlic. Mix well to combine.
Rub the chicken with the garlic herb butter. Pour half of the prepared garlic herb butter over the surface of the chicken. Use your hands to rub the butter and herbs over the entire surface and inside of the chicken. Set aside.
Prepare vegetables. Chop and prepare the yellow onions, carrots, butternut squash, and potatoes. Smash the remaining (10 cloves) of garlic with a large chef's knife. Add the onions to a large Dutch oven (at least 7.5 quarts) first, followed but the carrots, potatoes, butternut squash, and garlic. Season with salt and pepper.
Stuff and truss chicken. Stuff your chicken with a few carrots and the quartered lemon. Gently separate the skin from the breast meat and carefully stick the remaining butter under the skin. Tie the legs together using kitchen twine (don’t worry, looks aren’t important).
Transfer chicken to your Dutch oven. Transfer the seasoned chicken, breast-side-up, to the Dutch oven directly on top of the vegetables. Pour any remaining garlic herb butter over the vegetables and chicken.
Cook. Roast uncovered for 10 minutes at 475 degrees F. Reduce heat to 375 degrees F. and continue to cook for approximately 1 hour and 40 minutes (or approximately 20 minutes per pound), basting at least once or twice throughout cooking (see notes).
Rest. Once the chicken reaches an internal temperature of 165 degrees F as read with a digital meat thermometer remove from the oven and cover with foil. Allow chicken to rest 10 minutes before carving.
Serve. Serve drizzled with pan juices and cooked vegetables and garnished with fresh minced parsley.