Place potatoes, carrots, and onion wedges in the bottom of the slow cooker.
Season the roast with salt, pepper, and garlic powder, then place on top of the vegetables.
Sprinkle onion soup mix over the roast and pour beef broth around the sides.
Cover and cook on low for 8 hours or high for 5 hours until beef is fork-tender.
In a small bowl, whisk together cornstarch and cold water until smooth to create a slurry.
Turn slow cooker to high, stir the cornstarch slurry into the cooking liquid, and cook uncovered for 10-15 minutes until gravy thickens, stirring occasionally.
Slice or shred the roast and serve with vegetables and thick gravy.