Place mushrooms and onion in a lightly greased 5-to 6-qt. slow cooker.
Sprinkle roast with pepper. Cook roast in hot oil in a large skillet over medium-high heat 2 to 3 minutes on each side or until browned.
Place roast on top of mushrooms and onion in slow cooker. Sprinkle onion soup mix over roast; pour beef broth and tomato sauce over roast. Cover and cook on LOW 8 to 10 hours or until meat shreds easily with a fork.
Transfer roast to a cutting board; cut into large chunks, removing any large pieces of fat. Keep roast warm.
Skim fat from juices in slow cooker; stir in tomato paste and Italian seasoning. Stir together cornstarch and 2 Tbsp. water in a small bowl until smooth; add to juices in slow cooker, stirring until blended. Increase slow cooker heat to HIGH. Cover and cook 40 minutes or until mixture is thickened. Stir in roast.