1. Place mushrooms and onion in a lightly greased 5-to 6-qt. slow cooker.
2. 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.
3. 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.
4. Transfer roast to a cutting board; cut into large chunks, removing any large pieces of fat. Keep roast warm.
5. 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.
For an easy side, sauté 2 cups diced zucchini, 1 cup frozen sweet peas, and 1/2 cup diced onion in 3 Tbsp. olive oil until tender. Toss with 20 oz. hot cooked cheese-and-spinach tortellini,and 1/2 cup each shredded Parmesan cheese and chopped fresh basil. Season with salt and pepper to taste.