Preheat the oven to 300°F. Pat the beef dry and season it liberally with the kosher salt on both sides. Heat the oil in a large Dutch oven over medium-high heat. A splatter screen reduces clean up. When it shimmers, add the beef and cook undisturbed until the underside is deeply browned, about 10 minutes. Turn the beef and brown the other side, about 10 minutes more.
Using tongs, remove the meat from the pot and set it aside on a dinner plate. There should be just a film of fat in the pot; if necessary, pour out any excess. Reduce the heat to medium, add the onion and cook, stirring occasionally, until it browns, about 6 minutes. By now, the bottom of the pot will be darkened from the meat bits stuck to it. Add the flour and cook for 1 minute, stirring, until the flour is absorbed. Pour in the stock and Worcestershire sauce. While the liquid comes to a boil, use a wooden spoon to scrape the film of flour and the browned bits from the bottom of the pot. Add the salt, pepper, thyme, bay leaves, and the beef, along with any juices from the plate, to the liquid. The surface of the beef will be just above the liquid. Cover the pot securely and place it in the oven for 2 1/2 hours.
Meanwhile, cut the carrots and parsnips by dividing the slender part from the thicker end. Slice the slender end into 1-inch rounds; cut the thick ends lengthwise and crosscut them into 1-inch crescents. Cut the potatoes into 2-inch-long segments.
Turn the beef over and fit the carrots, parsnips, and potatoes around the beef, pushing them into the liquid so that they are partially submerged. Cover the pot and continue braising, stirring the vegetables once, until the beef shreds readily with a fork and the vegetables are very tender 1 to 1 1/2 hours more for a total of 3 1/2 to 4 hours.
Transfer the pot roast to a cutting board, scoop the root vegetables into a warmed serving bowl, and discard the bay leaves. If necessary, skim any fat from the gravy with a ladle. (If serving this the next day, chill the gravy in the refrigerator and lift off the layer of fat.) Stir in the mustard, if using, and taste for seasoning.
the natural seams, trim off the fat, and cut it into serving pieces. Arrange the beef in the center of the vegetables in the serving bowl and pour the gravy over all.
Source: Curry, Lynne. "Grassfed Pot Roast with Parsnips, Carrots, and Fingerlings." Pure Beef: An Essential Guide to Artisan Meat with Recipes for Every Cut. Pennsylvania: Running Press Book Publishers, 2012. 104-105. Print.