Remove the chicken from the refrigerator and let sit at room temperature for 1 ½ to 2 hours. Preheat the oven to 475°F.
Wash, cut, and prepare all of the vegetables for the vegetable nest. Combine the onions, celery, fennel, corn, tomatoes, carrots, sweet potatoes, brussels sprouts, and 3 garlic cloves in a bowl. Add 3 tablespoons olive oil, 1 teaspoon salt, and ¼ teaspoon pepper, stir to combine.
Spread the vegetables in a casserole dish or roasting pan. Add 5 sprigs of thyme, 3 sprigs rosemary, and lemon slices to the vegetables.
Prepare the chicken. Remove the neck and innards if they are still in the cavity. Using a paring knife, cut out the wishbone from the chicken (you can leave it in, it is sometimes harder to cut after roasting).
Season the cavity with ½ teaspoon salt and ¼ teaspoon pepper, add 3 garlic cloves and 5 sprigs of thyme, massage the inside of the bird to infuse it with the flavors. Add one lemon slice into the cavity.
In a small bowl combine butter and 1 tablespoon chopped rosemary. Carefully add about 1 tablespoon of the butter mixture beneath the skin to cover the breast. Repeat with the other side.
Truss the chicken or leave untrussed. Use the remaining rosemary butter to coat the outside surface. Season with salt and pepper.
Make a nest in the center of the vegetables and nestle the chicken in the center. Place the pan in the center of the oven and roast for 25 minutes.
Reduce the heat to 400 °F and roast for an additional 30 to 45 minutes, or until the temperature registers 160°F in the meatiest portion of the bird. The juices should run clear.
If needed, return the bird to the oven for more roasting, and check every 5 minutes. Transfer to the cutting board and let it rest for 20 minutes.
Just before serving, set the pan of vegetables over medium heat, turning and glazing them with the pan juices. Season with salt and pepper as needed.
If there are any additional pan juices, separate from the vegetables and make a quick sauce if desired.
To make a pan sauce heat a small saucepan over medium-low heat. Add about 1 tablespoon of butter and 1 tablespoon of flour. Whisk to create a roux and once it is a pale yellow, add the hot pan juices and continue to whisk until the sauce is slightly thickened. Season with salt and pepper.
Cut the chicken into pieces and arrange over vegetables, serve with pan sauce if desired.