At least 1 hour (or up to 24 hours) before cooking, pat the chicken dry and season inside and out with the salt and pepper. Refrigerate uncovered.
Heat the oven to 425°F (220°C). Arrange the onion wedges in a roasting pan or large ovenproof skillet.
Mix the lemon zest with the olive oil and rub over the chicken. Stuff the cavity with the lemon halves, half the garlic and the herb sprigs. Tie the legs together with kitchen string and tuck the wing tips under.
Place the chicken breast side up on the onions. Put the remaining garlic half beside it and pour the broth into the pan.
Roast for 60–75 minutes, until the skin is deep golden and a thermometer in the thickest part of the thigh reads 165°F (74°C) and the juices run clear.
Rest the chicken for 15 minutes before carving. Squeeze the roasted lemon from the cavity into the pan juices, stir in the parsley and spoon over the carved chicken.