Place sliced onions in the bottom of the Instant Pot or slow cooker
Rub the roast with salt, pepper and garlic and place on top of the onions
Add beef broth and Worcestershire sauce to the pot
If using an Instant Pot manually set to 90 minutes sealed. Let it naturally release for 10 minutes and then release the vent.
If using a slow cooker, do all of the previous steps but set on low for 10 hours
For the horseradish sauce, mix horseradish and mayonnaise together
Shred the meat with a fork and remove the cooking juices and put in single sized serving dishes for dipping
Toast the bread and layer each sandwich with shredded meat, onions and horseradish sauce. Served with a dish or au jus. Enjoy!