Heat vegetable oil in a large skillet over medium-high heat and sear the roast on all sides until deeply browned, about 2-3 minutes per side.
Transfer the seared roast to your slow cooker and sprinkle the onion soup mix over the top.
In a bowl, whisk together beef broth, balsamic vinegar, brown sugar, Dijon mustard, and minced garlic.
Pour the broth mixture over the roast in the slow cooker.
Cover and cook on low for 8 hours until the meat is fall-apart tender.
Remove the roast carefully as it will be very tender, shred with two forks, and serve with the cooking liquid.