Preheat the oven to 350 degrees F. Coat the inside of a flameproof (such as enameled cast iron)
9 1/2 by 12 by 4-inch baking dish with the butter.
Season the lamb chops with 1/2 teaspoon of the salt. In a large skillet, heat the oil over high heat. In batches, cook the lamb chops, turning once, until browned on both sides, about 4 minutes. Set aside.
In a large bowl, mix the potatoes, onion, thyme, garlic, pepper, and the remaining
2 teaspoons salt. Spread half of the potatoes in the dish. Arrange the lamb chops on top, then cover with the remaining potatoes. Pour the lamb stock and chicken stock into the dish. Bring to a boil over medium heat. Cover tightly with aluminum foil. Bake until the potatoes are very tender, about 3 1/2 hours. During the last 30 minutes, remove the foil so the potatoes can brown lightly.