In a small bowl, combine tomato sauce, brown sugar, oregano, horseradish, and mustard. Stir until well-mixed and set aside.
Use a paring knife to poke small cuts about 1½ inches deep all over your roast and insert sliced garlic into the slits.
Place roast in a large bowl or plastic bag and sprinkle with flour, salt, and pepper. Rotate the roast as needed until all sides are well-coated.
In a large Dutch oven with a lid, heat oil over medium-high heat. Once hot, add roast to the pot and brown on all sides.
Add onions to the pot around the roast and pour red wine over the top. Pour tomato sauce mixture over the roast and add the bay leaf. Cover the pot and bring the liquid to a simmer.
Once simmering, reduce the heat to keep the liquid at just a simmer and cook for 2½ hours.
Flip the roast over and then nestle the potatoes, carrots, and celery around the meat, making sure they’re mostly submerged.
Place the lid back on the pot, bring the liquid back up to a simmer, and cook for 1 hour more, until the vegetables and meat are tender. (You should be able to pierce both the potatoes and the meat easily with a sharp knife.)
(Optional) If you would like to skim the fat from the top of the sauce, transfer the roast to a cutting board and use a slotted spoon or spider skimmer to transfer the vegetables to a serving bowl. Let the liquid settle for 2 to 3 minutes, and then use a large spoon to skim the fat from the surface.
Taste the sauce, and add salt and pepper as needed. Serve the meat cut into chunks with sauce spooned over the meat and vegetables* and enjoy!