Reserve chicken giblets for other uses. Pull off and discard lumps of fat from chickens. Rinse birds inside and out and pat dry.
Sprinkle 1/2 teaspoon poultry rub in each bird's body cavity. Place 1/2 the onion, lemon, and parsley in each chicken. Pin body cavities shut with small metal skewers.
If using cheese, starting at the neck, gently push your fingers under skin of each breast to loosen skin. Spread 5 ounces cheese under skin of each chicken, pressing to form an even layer over breast.
Rub skin of each chicken all over with 1/2 the oil and 1/2 the remaining poultry rub. Set chickens, breasts up, at least 2 inches apart on a rack in an 11- by 17-inch roasting pan.
Roast in a 400° oven until a thermometer inserted through thickest part of thigh to joint registers 175°, about 65 minutes (about 55 minutes in a convection oven).
Using pot holders to protect your hands, tilt chickens to drain juices into pan. Set chickens on a platter and let rest in a warm place 10 to 15 minutes. Stir pan juices to release browned bits; skim and discard fat, and pour drippings into a small bowl. Carve chicken and serve with juices.