Cook Beef
Season roast with salt and pepper.
Sear roast in oil until browned on all sides.
Combine roast, sliced onion, garlic, Worcestershire sauce, soy sauce, thyme, garlic powder, onion powder, and pepper.
Cook on LOW for 6-8 hours or HIGH for 3-4 hours until tender.
Caramelize Onions
Cook sliced yellow onions in butter with salt and sugar over medium-low heat until deeply golden.
Make Horseradish Sauce
Combine mayo, sour cream, horseradish, lemon juice, chives, salt, and pepper.
Prepare Au Jus
Strain beef cooking liquid into a saucepan.
Simmer liquid until reduced.
Stir in cold butter.
Assemble Sandwiches
Toss shredded beef with some au jus.
Toast rolls with butter.
Spread horseradish sauce on rolls.
Top with beef, caramelized onions, and provolone cheese.
Broil until cheese is melted.
Serve with au jus for dipping.
