Preheat oven to 350°F (175°C). Heat oil in a dutch oven or heavy based oven-proof pot over medium-high heat. Season roast all over with a good amount of salt and pepper. Sear until brown on all sides (about 4-5 minutes each side).
Transfer roast to a plate. Sauté onions until transparent, then add garlic and cook for 30 seconds until fragrant. Add the stock and balsamic vinegar to deglaze your pan, scraping up any browned bits. Whisk in the flour and let cook for about 4 minutes (don't worry about any lumps, they will cook out).
Transfer roast back into the pot. Add the potatoes, carrots, celery, mustard, brown sugar, thyme and bouillon. Season with salt and pepper to taste.
Bring to a simmer, cover with lid (or foil) and transfer to the oven. Roast for 3-4 hours until the meat is tender and falling apart. (Check roast after 1 1/2 hours. If the liquid has mostly been absorbed, stir in 1 cup of extra broth and continue cooking.)
Transfer the roast, carrots, and potatoes to a warm plate. With a spoon, skim the fat off the surface of the cooking liquid. Cut the roast into thick slices, and serve with the vegetables. Pass the pan juices separately.