For the brisket: Add half of the sliced onions to a slow cooker. Sprinkle the brisket on one side with salt and pepper and add to the slow cooker, seasoned-side down. Sprinkle the other side with salt and pepper. Top with the remaining onions and the soup mix and Worcestershire.
Cover and cook on high for 6 hours. Let the brisket sit in the onion juices on warm for 30 minutes, or until ready to serve.
For the white barbecue sauce: Whisk together the mayonnaise, vinegar, hot sauce, garlic powder, 1/2 teaspoon salt and a generous amount of pepper. Refrigerate until ready to serve.
To assemble: Cut the brisket in half and slice thinly against the grain. Top the potato buns with brisket, onions and barbecue sauce.