Trim fat from meat. If necessary, cut roast to fit into a 3-1/2- to 6-quart slow cooker. Lightly coat a large skillet with cooking spray; heat over medium heat. Brown meat on all sides in hot skillet. Place meat in the prepared cooker.
Clean mushrooms; remove and discard stems. Cut mushrooms into 1/4-inch slices. Add to cooker. Pour broth over meat and mushrooms.
Cover and cook on low-heat setting for 8 to 9 hours or on high-heat setting for 4 to 4-1/2 hours. Remove meat from cooker; cover and let stand for 10 minutes.
Meanwhile, using a slotted spoon, remove mushrooms and set aside. Thinly slice meat. Arrange meat, mushroom slices, and onion slices on toasted buns. Pour cooking juices into a measuring cup; skim off fat. Drizzle a little of the juices onto each sandwich and pour the remaining juices into bowls to serve with sandwiches for dipping. Makes 8 sandwiches.