Preheat the oven to 400F degrees. Rinse the chicken and pat dry. Place the chicken in a large roasting pan. Lift the skin from around the breast and thigh (without tearing it) and slip garlic slices between meat and skin.
Slide 1 rosemary sprig under the skin on each breast and place remaining 2 sprigs in the bird's cavity. Halve the lemon and squeeze the juices over the chicken. Place the lemon halves in the cavity. Season the skin and cavity well with salt and pepper to taste.
Roast the chicken, basting occasionally, until the leg juices run clear when pierced with a knife, about 1 1/2 hours.