Pat your pot roast dry, spread a very thin layer of mustard on it, and season well with salt, pepper, garlic powder, and smoked paprika on all sides. If the roast is not the same thickness all the way through or falling apart, tie it with butcher's twine.
If you want to, brown the roast on all sides in a skillet before adding it to the slow cooker. To do this, place a skillet over medium heat and add the 2 tablespoons of olive oil.
When your skillet and olive oil are hot, sear your roast in a pan on all sides, about 3-4 minutes per side until you get a nice browning on all sides.
Wash and chop your onions, potatoes, carrots, and celery into pieces that are slightly larger than bite sized.
Place the onions, carrots, potatoes, garlic, and celery (if using) in the slow cooker along with the wine, beef broth or stock, Worcestershire sauce, thyme, rosemary, and bay leaves. Add in the roast, nestling it between the veggies and cook in the crockpot for 6-7 hours on high.
When the meat shreds easily, remove the butcher's twine and shred.
For a thicker gravy, make a cornstarch slurry from 2 tablespoons of cornstarch and 4 tablespoons of water and mix it with the cooking liquid in a small sauce pan on the stove top. Boil over medium heat until the cornstarch is cooked and the gravy's consistency is to your liking.
Pour the gravy back in your slow cooker over the veggies and shredded meat or serve on a serving platter. Garnish with fresh chopped parsley and enjoy!