Spray slow cooker with nonstick cooking spray. Rub roast with salt and pepper.
Place onions at bottom of slow cooker. Add beef broth, Worcestershire sauce, and garlic. Toss to combine. Place beef on top.
Cook on high for 6 hours or low for 8 hours.
Heat broiler. Spread butter on each side of hoagie buns, place buns on a cookie sheet, and put under broiler. Broil 2 minutes or until butter has completely melted into the hoagies. Remove from oven, but keep on pan and keep broiler on.
Spread horseradish sauce onto hoagies. You can do one or both sides depending on how much you like horsey sauce.
Remove beef from slow cooker and place onto cutting board. Strain liquid from slow cooker to a bowl, reserving onions.
Cut roast horizontally in half, then crosswise into slices. Top buns with slices of beef, then onions, and Gruyere. Put under broiler again and broil until cheese melts.
Serve warm with au jus for dipping.
Recipe by Simple, Sweet & Savory at https://www.simplesweetsavory.com/slow-cooker-french-dip-sandwiches/