Season the roast with salt and pepper. In a large Dutch oven over medium-high heat, add the oil. When the oil is hot, add the roast and sear on all sides until well browned, about 5 minutes. Remove to a plate.
Add the celery, carrots and onions and saute until caramelized on the edges, about 5 minutes. Add the flour, garlic, thyme, bay leaves and rosemary. Stir to incorporate the flour into the oil and cook until the flour smells toasty, about 2 minutes.
Pour in the wine and bring to a boil. Add the roast back to the pot, along with the stock and potatoes. Bring to a rapid simmer and cover tightly. Place in the oven and cook, covered, until the meat is just tender (a knife will slide out easily with no resistance), about 2 hours 30 minutes. Remove the meat to a cutting board.
Let the sauce sit for a few minutes, and then spoon off any fat that has risen to the surface (or pour into a fat-separating measuring cup, pour off the fat and add the sauce back to the pot). Return the sauce to a simmer. Whisk in the mustard and horseradish and season with salt and pepper if necessary. Discard the bay leaves and stir in the parsley if using.
Carve the meat into 1/2-inch-thick slices against the grain, and serve with the sauce and vegetables.