Pat the beef chuck roast completely dry with paper towels. Season generously on all sides with 1 tsp kosher salt and 1/2 tsp freshly cracked black pepper.
Heat olive oil in a large heavy skillet or Dutch oven over medium-high heat until shimmering. Sear the chuck roast for 4 to 5 minutes per side until a deep, dark golden-brown crust develops. Transfer the seared roast directly into the slow cooker basin.
Reduce skillet heat to medium. Melt the butter and add the sliced yellow onions with the remaining 1/2 tsp salt. Cook for 8 to 10 minutes, stirring occasionally, until the onions soften and begin turning golden brown around the edges. Stir in minced garlic and cook for 1 minute until fragrant.
Pour in beef bone broth, Worcestershire sauce, balsamic vinegar, and Dijon mustard, using a wooden spoon to scrape up all flavorful browned bits from the bottom of the skillet. Bring to a gentle simmer.
Pour the warm caramelized onion mixture over the seared chuck roast in the slow cooker. Tuck in the fresh thyme, rosemary sprigs, and bay leaves.
Cover and cook on LOW for 8 to 9 hours (or HIGH for 5 to 6 hours) until the beef is fork-tender and pulls apart effortlessly.
Carefully transfer the beef to a cutting board and shred into large succulent chunks, discarding the bay leaves and herb stems.
Stir the cornstarch slurry into the bubbling onion gravy in the slow cooker. Cover and cook on HIGH for 10 to 15 minutes until the gravy thickens into a rich, velvety glaze.
Return shredded beef to the slow cooker and toss gently to coat in the luxurious caramelized onion gravy. Serve warm over creamy garlic mashed potatoes or alongside crusty artisan sourdough bread.