Place roast into your slowcooker.
In a bowl mix together, 1 cup beef both, onion, garlic, dijon mustard, worcestershire sauce, garlic powder, onion powder and the salt & pepper.
Pour mixture over the roast. Add 2 bay leaves to the slow cooker. Cover and cook on Low for 8-10 hours, or High for 4-5 hours.
Remove bay leaves and discard. Remove roast to a large serving platter. Cover to keep warm.
Strain remaining beef juices into a saucepan. Add remaining 2 cups of beef broth. Season with salt & pepper. Cook on medium-high until it comes to a boil.
Mix the water and cornstarch in a small dish. Add cornstarch mixture to the beef broth. Stir for about 1 minute, or until the gravy has thickened. Serve roast beef with the gravy.