Use a sharp knife to trim as much of the fat as possible from around the edges of the roast. It’s fine if some fat remains but trim as much as you can to ensure that your gravy will not be overly greasy. Blot the beef dry with paper towels.
Combine the seasoning mix ingredients in a small bowl. Sprinkle the rub over all sides of the beef and use your hands to press the seasoning into the meat.
Heat the oil in a large skillet oven over MEDIUM-HIGH heat. Add the seasoned roast and sear it on all sides until nicely browned. Remove the pot from the heat and transfer the roast to a 6 quart slow cooker.
Return the skillet to MEDIUM heat and add additional olive oil, if necessary. Add the diced onion and minced garlic. Cook and stir until softened, about 3 to 4 minutes, and then add the tomato paste, stirring until well combined. Add the red wine (or ½ cup broth) and cook, stirring to deglaze the pan for just a minute or two. Pour the contents of the skillet into the slow cooker over and around the roast..
In a small bowl or measuring cup, whisk together 2 cups beef broth, Dijon mustard, and Worcestershire sauce. Pour the mixture around the roast.
Tuck the bay leaf into the liquid then scatter the carrots and potatoes around the roast.
Cover and cook on LOW for 7 to 8 hours or on HIGH for 5 to 6 hours.
When the roast is fork tender, transfer it to a cutting board and use two forks to pull it apart into chunks. Use a slotted spoon to transfer the carrots and potatoes to a serving dish with the beef.
For the gravy, pour the liquid from the slow cooker through a fine mesh strainer into a medium saucepan. If you’d like to remove some of the fat from the drippings, let the liquid rest for a few minutes and use a spoon to skim off any excess from the surface or use a gravy separator.
In a small bowl, whisk together the cornstarch and water until smooth. Place the saucepan over MEDIUM-HIGH heat and stir in the cornstarch slurry. Bring the mixture to a low boil, stirring frequently, then reduce the heat and simmer until thickened. Taste and season with additional salt and pepper, if needed.