Preheat the oven to 450 degrees F. Season the short ribs with salt and pepper. Place the ribs in a roasting pan that's just large enough to hold them in a single layer. Roast for 30 minutes. While this occurs, place the wine, stock, flour, sugar and mustard in a medium-sized pot and whisk until well combined. Mix in the onion, garlic and thyme. Bring to a boil, boil 2 minutes, and then remove from the heat. When the short ribs have roasted 30 minutes, remove from the oven and drain away the excess fat. Reduce the oven temperature to 325 degrees F. Pour the wine/stock mixture over the ribs. Cover and bake for 2 hours, or until the meat is very tender and falling off the bone. Skim any fat from the surface of the ribs before serving.