• Heat a large skillet over medium-high heat. Pat the beef dry and season all sides evenly with salt. Add the oil to the hot skillet and sear the roast for 1 to 2 minutes per side until well browned. Transfer the beef to a large slow cooker and sprinkle with black pepper.
• Place the skillet back over medium heat. Add the water, beef broth, and bouillon, stirring to loosen the browned bits from the pan. Once dissolved, pour the mixture over the beef in the slow cooker.
• Add the bay leaf, garlic, and mushrooms around the roast. Sprinkle the onion soup mix over the top, allowing some to fall into the liquid. Finish by adding the thyme and rosemary.
• Cover and cook on LOW for 8 to 10 hours, until the beef is very tender and slices easily. For a quicker start, you may cook on HIGH for 1 hour, then reduce to LOW for the remaining time.
• Transfer the roast to a cutting board and rest for a few minutes. Slice thinly against the grain for the best sandwich texture. Keep warm.
• Preheat the broiler and place rolls cut side up on a baking sheet. If desired, mix butter with garlic powder and spread lightly on the rolls. Broil until lightly golden, flip briefly to crisp the outside, then return cut-side up and add cheese. Broil just until melted.
• Spread horseradish sauce on the melted cheese, add sliced beef and mushrooms, and close the sandwiches. Serve hot with small bowls of the cooking jus for dipping.

Notes:

• Make-ahead tip: Cook the beef and refrigerate it in the jus overnight. Reheat gently before assembling.
• Use sharp provolone for extra flavor, or Monterey Jack for a creamier melt.
• To make gluten-free: Use your favorite GF sandwich rolls.