Preheat oven to 425Allow the ingredients, chicken included, set out to become room temperature.
In a large Dutch oven or roasting tray with a lid, drizzle one tablespoon of olive oil into the pan and brush around the bottom and sides.
Scrub the potatoes and carrots. On a cutting board with a sharp knife, rough cut the unpeeled potatoes and carrots into rustic chunks. Set aside.
Place chicken/chickens, into the large roasting pan. To obtain a really moist texture to the chicken breast meat place the chicken in the pan breast side down, this allows the breast to roast in the pan juices.
Sometimes chickens are sold with an instant-read thermometer inserted, if not, set your meat thermometer out because we will use it later, as we will want the internal temperature of the deepest part of the chicken to register at 165.
Take several slices of onions and place into the cavity of the chicken, which helps flavor the meat close to the bones. Arrange the potatoes around the bottom of the pan, around the chicken. Drizzle a little olive oil across the top of the chicken and potatoes. Sprinkle a little salt, pepper and some of the herb blend across the chicken and potatoes. Layer the remaining onions, carrots and Brussels sprouts on top of the potatoes. Bury the lemon halves into the veggies and sprinkle the remaining salt, pepper and herb blend.
In a small pan on top of the stove, melt the herb butter/regular butter. Drizzle the melted butter gently across the chicken skin, around the outside of the chicken and across the vegetables.
Place the pan and its ingredients, uncovered, into the preheated, high temperature oven and roast for 30-minutes.Lower the oven to 375, cover and continue to roast until the chicken registers 165 internal temperature, about 90-minutes..Remove the cover, turn off the oven and let it rest in the oven another 15 minutes, which will help to create a golden brown, crispy skin.Total roasting time will be about 90-minutes, though cooking time will vary depending on several variables, so check the internal temperature to be certain the poultry is fully cooked.