To cook the roast:

1. Generously salt and pepper both sides of the roast, and heat the olive oil over medium high heat in a large, heavy bottomed skillet until shimmering. You should be able to see a few wisps of smoke come off of the surface.
2. Add the seasoned roast to the hot skillet and sear each side for 6 minutes. Remove the roast from the pan, add to a 6 quart slow cooker and arrange the cut sweet potatoes around it.
3. Keep the skillet over medium high heat and add the onion, celery, carrot and garlic to the hot beef fat. Cook, stirring occasionally, until the veggies have cleaned the browned bits from the bottom of the pan and have absorbed the beef fat. This should only take a few minutes. Add the veggies to the slow cooker and return the skillet to medium high heat.
4. Separate out 3 TB of beef broth from 1 cup and set aside. Combine the remaining beef broth, balsamic vinegar, Dijon mustard and brown sugar in the hot skillet and whisk to dissolve the sugar. The mustard will still be a bit chunky and not blended in. Bring the liquid to a simmer and continue to whisk until the mustard has blended in and the sauce is smooth.
5. Pour the sauce over the beef, potatoes and veggies. Cover the slow cooker and cook on high for 5 hours.

To make the gravy and serve:

1. After the roast has finished cooking, remove the beef and potatoes from the slow cooker and arrange on a serving platter.
2. Strain the leftover cooking liquid through a colander or fine sieve to separate out the veggies. Add the veggies to the sweet potatoes on the platter, and pour the liquid into a small saucepan.
3. Bring the liquid to a boil over medium high heat. While you wait for the liquid to boil, stir the 3 TB of beef broth into the flour, thyme, salt and pepper to form a runny paste.
4. When the liquid comes to a boil, stream in the flour paste and whisk constantly until the gravy has thickened. Remove from the heat.
5. Serve roast with the sweet potatoes, veggies and gravy. Refrigerate any leftovers for up to 3 days.