Season the chuck roast with salt and pepper. Place in the slow cooker, adding sliced onions if desired. Pour over beef stock to cover. Set slow cooker to low and cook for 6-8 hours until tender.

Check the beef after 6-8 hours; it should be fork-tender. Remove the beef and let it rest before shredding.

Peel and chop potatoes. Boil in salted water for 15-20 minutes until tender. Drain and mash with warm milk and butter until creamy.

Make gravy using drippings by creating a roux with butter and flour over medium heat. Gradually whisk in beef stock, then return to slow cooker until thickened.

Assemble the sandwich with a slice of bread, layers of shredded beef, mashed potatoes, and topped with gravy.

Serve with optional garnishes like steamed vegetables or a cherry on top.