Sprinkle roast evenly with salt and pepper. Heat oil in large skillet or Dutch oven on medium heat. Sear roast on all sides. Place roast in slow cooker and top with peppers. Add 1/2 cup stock to skillet, stirring to release browned bits from bottom of pan; transfer to slow cooker.
Add Italian seasoning, garlic powder, vinegar and remaining beef stock to slow cooker. Cover.
Cook 8 hours on LOW or 4 hours on HIGH.
Meanwhile, mix cornstarch and water in a small bowl with wire whisk; set aside. Remove roast from slow cooker. Stir cornstarch mixture into au jus. Cover and cook 30 minutes on HIGH.
Slice beef into thin strips across the grain. Return beef to au jus in slow cooker. Serve beef and peppers on toasted Italian roll or baguette with au jus on the side for dipping.
Test Kitchen Tip: The Italian Beef is a sandwich native to Chicago. Similar to the French Dip, it is made with thinly sliced seasoned roast beef, served on a crusty Italian-style roll, and then “dipped” or completely dunked in au jus. It can be served “sweet” topped with grilled peppers, or “hot” topped with Giardiniera, an Italian blend of pickled peppers and vegetables.