Season the beef with the salt and pepper and brown it in 1 tbsp oil over medium-high heat, 5 minutes per broad side.
Transfer the beef to a 6-quart slow cooker, add the remaining oil to the skillet and cook the onions until golden at the edges, 8 minutes.
Add the onions, broth, wine, Worcestershire, thyme and bay leaf to the cooker.
Cover and cook on low until the beef pulls apart easily, 7 1/2 to 8 hours.
Remove the beef, discard the bay leaf and skim the cooking liquid thoroughly.
Shred the beef, moisten it with 1 cup liquid and strain the remaining jus into small bowls.
Toast the split rolls, fill them with beef and onions and melt one Swiss slice over each under the broiler.
Serve the sandwiches with the hot strained jus.