Add the potatoes, carrots, shallots, 1 teaspoon of the salt, and ¾ teaspoon of the pepper to a 6-quart slow cooker.
Using the tip of a paring knife, make deep narrow slits all over the roast, and insert 1 piece of garlic deep into each slit. Season the roast all over with the remaining 2 teaspoons salt and ¾ teaspoon pepper. Transfer the roast to the slow cooker and nestle it among the vegetables if space allows. Otherwise, let it rest on top.
In a small container, whisk together the beef stock, cognac, Worcestershire sauce, mustard, and lemon juice. Pour this into the slow cooker.
Cook on high for 7 hours, until the meat is tender.
Combine the butter and flour in a small bowl, and mash with the back of a spoon or a small spatula until uniform to make a beurre manié. During the last 10 minutes of cooking, transfer the roast from the slow cooker to a platter and set aside. In three parts, whisk the beurre manié into the jus left in the cooker until incorporated. Stir in the ½ cup chipped parsley. Carve the roast into the desired portions, or slice it thinly, and return the meat and the vegetables to the slow cooker.
Serve the pot roast, vegetables, and hot sauce topped with additional chopped parsley, fleur de sel if desired, and freshly ground black pepper. Don’t forget the crusty bread for mopping up the delicious sauce!