Rearrange oven racks so that the bird can be placed in the center of the oven. Preheat oven to 475 degrees F.
Discard giblets (and any other foreign objects) that you find inside the bird. Rinse bird under cold water, then pat dry. Rub generously with salt, inside and out.
In a small bowl, blend butter, pepper, rosemary, garlic and lemon zest. Rub the butter mixture all over the birds. Use your fingers to separate the skin from the breast meat and slide some of the mixture underneath the skin. Cross the legs and bind with twine.
Place potatoes, cut side down in a large shallow roasting pan, or some facsimile. Perch bird atop potatoes. Roast for 30 minutes. Remove to baste and squeeze juice of 1/2 lemon overtop. Reduce temperature to 375 degrees F. Roast for 60 more minutes, or until the juices of the thigh meat run clear when pricked. If bird begins to burn on top, cover loosely with a sheet of aluminum foil.
Squeeze other lemon 1/2 overtop, then cover tightly with aluminum foil, and let sit at room temperature for 5 minutes before serving (this locks in the juices).