Peel the potatoes and cut into pieces, 2 inches in diameter. Place in a pot with salted water to cover, bring to a boil, and parboil for 7 minutes. Drain and return the potatoes to the heat to evaporate any remaining water. Add salt, pepper, and semolina. Cover the pot and shake several times to rough up the surface. (This will produce a crisper result.)

Meanwhile, preheat the oven to 450° and heat the duck fat in a roasting pan just large enough to hold the potato pieces in a single layer. Add the potatoes to the pan and roast for 15 minutes. Turn the potatoes, baste, and roast for 15 minutes more. Turn the potatoes, add the thyme, baste, and roast for 15 minutes more. Add salt if desired and serve immediately.