Heat olive oil in your slow cooker. While the oil is heating, generously season all sides of the chuck roast with freshly ground pepper.

When the oil starts to shimmer, add onion and let it cook for 5-10 minutes. Carefully place the roast and the rest of the ingredients. Cook on low for 6 hours.

Transfer the beef to a plate and shred with a fork.

Reserve 3 cups of the juice from the slow cooker and add to a small sauce pan. Bring it to a boil over medium-high heat. Reduce to a hard simmer and let it cook until reduced by half, about 10 minutes.

Using butter knife, spread butter outside of roll to create crispy top and bottom.

Flip the roll on the other side and spread horseradish, top it with cheese. Add a pile of shredded beef. Place other slice of bread over the top, with the buttered side on top. Grill Panini sandwich with the lid close for about 5 minutes, or until cheese melts. Serve immediately with the sauce on the side for dipping.