Preheat the oven to 425 degrees F. Place the roast in a roasting pan; season with salt, pepper and thyme. Roast 20 minutes, and then reduce heat to 325 degrees F. Roast 40 to 50 minutes more for medium rare, 60 for medium and 70 for well-done. To make the potatoes, place with the saffron in a pot and cover with 2-inches of cold water. Gently boil until very tender, and then drain the water away through a sieve. (The cooking liquid can be cooled and later used in breads and stock.) Place the potatoes and any saffron threads caught in the sieve back in the pot and thoroughly mash. Whip in the buttermilk and butter; season with salt and pepper. Keep warm until the roast is ready. When cooked to desired doneness, transfer the roast to a platter. Place the roasting pan on the stove over medium-high. Add the wine and simmer and reduce by half. Whisk the stock and flour together in a bowl until lump free. Whisk into the pan, bring to a simmer and cook until lightly thickened. Thinly slice the beef and serve the sauce and potatoes alongside.