Season the roast generously with salt and pepper on all sides. Lightly coat the roast with flour on all sides.

If the slow cooker insert has a sear function, preheat it. Otherwise, heat a heavy pan, such as a cast iron skillet or Dutch oven, over medium-high heat on the stove.

When the insert or pan is hot, add the olive oil and sear the roast for about 3-4 minutes per side until it is evenly browned on all sides. Set it aside.

In the insert of a slow cooker, mix beef broth, Worcestershire sauce, sliced onions, and bay leaf. Add the beef to the slow cooker and cook on HIGH for 3-4 hours or LOW for 6-8 hours until the meat is tender.

Transfer the roast to a cutting board. Slice the beef into 1/4”-thick slices.

Strain the broth and discard the bay leaf and any remaining thyme sprigs. Return the onions and sliced beef to the slow cooker. Add just enough broth to keep the meat moist. Keep the reserved jus warm in a separate saucepan, seasoning to taste with salt and pepper, if necessary.

Toast your rolls.

While the rolls are toasting, whisk together the mayonnaise and horseradish. Spread a thin layer on the toasted rolls.

Top with slices of beef, some onions, and cheese, and place under a broiler to melt. Serve with reserved jus.