Combine the salt, cumin, paprika, garlic powder, black pepper, cayenne and lemon zest in a bowl. Wash the chicken and pat very dry. Rub the spice mixture all over the chicken. Set the chicken on a plate and refrigerate, uncovered, for at least 1 hour and up to 8 hours (to dry out the skin and promote crisp skin).
When you are ready to cook, preheat the oven to 425 degrees F.
Stuff the chicken with the lemon quarters, garlic and bay leaves. Arrange the onion slices to make a bed for the chicken in a large cast-iron skillet. Set the chicken on the onions and tuck the wings tips under the chicken's body. Brush the chicken all over with the melted butter.
Roast the chicken, basting once halfway through with the pan juices, until the skin is crisp and golden and an instant-read thermometer inserted in the thigh (do not touch bone) reads 165 degrees F, about 45 minutes. Let rest at least 10 minutes before carving.