Preheat the oven to 475°. Combine the salt, pepper and rosemary, and press the mixture firmly into the pork roast. Place the seasoned pork, fat side up, in a heavy casserole just large enough to hold the pork. Roast uncovered on the middle shelf of the oven, turning the meat over 2 or 3 times, for 20 to 30 minutes, or until the pork is lightly browned. Then reduce the oven to 325°. Scatter the chopped onions, carrots and the bouquet garni around the pork, cover the casserole, and roast for 40 minutes longer, basting the pork every 10 minutes with the juices that accumulate in the pan.
Meanwhile butter the bottom and sides of a shallow baking-and-serving dish about 12" long and 1-1/2" to 2" deep. Melt 2 tablespoons of butter in an 8" to 10" skillet, and in it cook the onions over moderate heat, stirring frequently, for 10 minutes, or until they are limp and lightly colored. When the pork is ready, spread the potato slices over the bottom of the baking dish and season with salt and pepper. Scatter the onions over the potato slices and pour in the heated stock. Place the pork loin on top and moisten both the pork and the potatoes with 2 or 3 tablespoons of fat skimmed from the drippings in the casserole. Roast uncovered in the bottom third of the oven for another 1-1/4 hours or until the pork is crusty and glazed, and the potatoes and the onions are tender and brown.
While the pork is roasting, add 1-1/2 cups of stock to the drippings left in the casserole. Bring to a boil over high heat and cook for 5 minutes, stirring frequently and scraping in any browned bits that cling to the bottom and sides of the pan. Strain the mixture through a fine sieve into a small saucepan, pressing down hard on the vegetables with the back of a spoon before discarding them. Let the liquid settle for a few minutes, then skim off the surface fat. Taste and season with salt and pepper. Set aside in the pan.
To serve, transfer the roast to a carving board, cut off the strings and slice the roast into 8 chops. Reheat the sauce made from the casserole drippings. Return the chops to the baking dish and arrange them in a line over the potatoes and onions. Moisten the pork with a little sauce and dust the potatoes with parsley. Service the remaining sauce in a sauceboat.