Remove wing tips (save for stock) and cut out wish bone. Preheat the oven to 500 degrees. Remove giblet package (save all but liver for stock pot). Pat the capon dry. Remove fat from tail and neck of chicken. Stuff cavity with herbs and garlic cloves. Season skin with salt and immediately set roaster in pan and set in oven.
After 15 minutes, remove the pan from the oven (making sure to close oven door), give bird a shake so it does not stick to bottom of pan and return to oven for 30 minutes. Meanwhile, scrub potatoes clean, leave skin on and cut, lengthwise into quarters so you have long potato wedges. Remove bird from oven again (making sure to close oven door) and scatter however many potatoes will fit on bottom of pan; set those which don't fit in an iron skillet with 1/8th inch of water on the bottom of skillet.
Return bird and potatoes to oven and roast for an hour more. Remove bird from oven and transfer to carving board; let rest for 20 minutes. Strain out juices and discard fat. Return juices to roasting pan. Scoop herbs and garlic out of bird cavity and add to roasting pan. Reduce juices by half. Season with salt and pepper. With slotted spoon, remove herbs and garlic. Right before serving whisk in butter and parsley.
Slice legs and breast and transfer to platter. Surround pieces of poultry by roasted potatoes. Serve gravy separately; serve also with salad on the same plate. After dinner, remove all skin and fat from what remains of bird. Remove flesh from the bones and store, tightly wrapped in plastic, in the refrigerator. Keep bones for a quick stock.