Season the chuck roast generously with salt and pepper on all sides.
Heat olive oil in a large skillet over medium-high heat and sear the roast for 3-4 minutes per side until deeply browned.
Transfer the roast to the slow cooker and add sliced onions and minced garlic around it.
In a bowl, whisk together balsamic vinegar, beef broth, brown sugar, and thyme, then pour over the roast.
Cover and cook on low for 8 hours or high for 4-5 hours until the meat is fork-tender.
Remove the roast and let rest for 10 minutes before slicing, then serve with the reduced cooking liquid spooned over the top.