Preheat the oven to 425F (220C) with the top oven rack in the middle of the oven.
Remove any gizzards and the neck from the cavity of the chicken. Transfer the chicken to a plastic (washable) cutting board, pat it dry with a paper towel, and remove any excess fat or skin from the chicken –this last step is optional and it depends on the chicken, but do not trim too much to expose meat from underneath.
Season the chicken:
Generously season with salt and pepper the outside skin and the cavity of the chicken. With a basting brush or your hands, brush the outside of the chicken with melted butter or olive oil and distribute it all around the skin.
Stuff the chicken with rosemary, halved lemon, and roughly cut garlic head. Place the chicken inside a 9×13 baking dish (with sides)
Add the vegetables:
Chop your vegetables over a clean surface. Place them into a large bowl, and season with olive oil. Toss to coat and transfer the veggies to the baking dish and distribute them around the chicken.
Roast:
Roast the chicken for 1 hour and 15 minutes (a little more for larger chickens, a little less for smaller), checking for doneness around the breast at 165F (75C) and 175F (80C) around the thigh. After 50 minutes, if the chicken is getting too browned, cover with foil for the remaining cooking time.
Remove the chicken from the oven and allow it to rest for 5-10 minutes on a platter or cutting board. Transfer the roasted vegetables to a serving platter and serve.