Preheat the oven to 375 degrees F. Wash the capon (actually a castrated male chicken) under cold running water, and soak for 1 hour in a pan of cold water in which 1/4 cup kosher salt has been dissolved. Drain and rinse again, using paper toweling to dry the bird inside and out.
Season the capon inside and out with a generous amount of salt and pepper. Put half the rosemary and thyme and all of the chopped garlic inside the cavity of the capon, along with some of the onion, carrot and celery. Tie the legs of the capon together to aid in even roasting and place, breast side down in a roasting pan. Rub the remaining herbs on the outside of the capon, drizzle all the olive oil over the capon, scattering the rest of the vegetables (except the potatoes) in the pan along with the capon. Place the roasting pan into the oven and roast the capon for 20 minutes on its breast, before turning onto its back for an additional 20 minutes. When the capon has roasted for 20 minutes on its back, add the potatoes and continue roasting for an additional hour or until the bird is thoroughly cooked (a meat thermometer registers 165 degrees F at the leg joint). Adjust seasoning with salt and pepper and remove from the oven, allowing the capon to rest for 15 minutes before serving.