Pat the roast dry and season generously with salt and pepper.
In a skillet, heat olive oil over medium-high heat. Sear the roast on all sides until browned, about 2–3 minutes per side.
Transfer roast to the slow cooker. Add sliced onions, mushrooms, and garlic on top.
In a small bowl, whisk together beef broth, Worcestershire sauce, balsamic vinegar, Dijon mustard, thyme, and rosemary. Pour over the roast and veggies.
Cover and cook on LOW for 8–9 hours or HIGH for 5–6 hours, until beef is fork-tender.
Optional: For a thicker gravy, remove ½ cup of liquid from the slow cooker, mix with cornstarch slurry, then stir back in. Let cook another 10–15 minutes.
Shred or slice the roast and serve with the rich mushroom-onion sauce. Garnish with fresh parsley.
