Tie the beef with kitchen twine if needed. Combine the 2 teaspoons salt, 2 teaspoons black pepper, and 1 tablespoon dried oregano in a small bowl and rub the mixture generously over the beef on all sides.
Heat the 1 tablespoon olive oil in a large skillet over medium high heat. Sear the beef on all sides until a deep brown crust forms.
Add the 2 sliced onions, 10 cloves minced garlic, 2 bay leaves, ¼ cup Worcestershire sauce, ¼ cup white wine, and 3 cups beef broth to the slow cooker and stir. Nestle in the beef and spoon some of the liquid over it.
Splash a little of the broth into the hot searing skillet and scrape up the browned bits, then add that to the slow cooker.
Cover and cook on HIGH for 2 to 3 hours or LOW for 4 to 6 hours, until an instant read thermometer reads 130 to 135°F for medium rare or 145°F for medium.
Transfer the beef to a cutting board, tent with foil, and rest for 10 to 15 minutes.
Strain the liquid from the slow cooker through a fine mesh strainer, reserving both the liquid and the onions. Discard the bay leaves. Let the liquid settle for a minute, then spoon the fat off the top. Taste and adjust the seasoning.
Slice the beef as thin as possible, against the grain.
Toast the cut sides of the rolls under the broiler. Pile on the beef, add the reserved onions and a slice of Swiss cheese if using, then return to the broiler for a minute or two until the cheese melts.
Serve immediately with the au jus in individual bowls for dipping.