01 - Rub the beef chuck roast thoroughly with kosher salt and black pepper on all sides.
02 - Arrange sliced onions and minced garlic evenly across the bottom of the slow cooker. Position the seasoned beef roast directly on top of the vegetables.
03 - Pour the beef broth and Worcestershire sauce over the beef and vegetables, ensuring the meat is partially submerged.
04 - Cover and cook on low setting for 8 hours until the beef becomes very tender and easily shreds with a fork.
05 - About 40 minutes before beef finishes, place cubed potatoes in a large pot and cover with cold water. Add a generous pinch of salt, bring to a boil, and cook for 15-20 minutes until fork-tender. Drain thoroughly, then mash with butter, milk, salt, and pepper until smooth and creamy.
06 - Transfer the cooked beef from the slow cooker to a cutting board. Use two forks to shred the meat into bite-sized pieces. Strain the cooking liquid and reserve 1 cup for gravy preparation.
07 - Melt butter in a saucepan over medium heat. Whisk in flour and cook for 1-2 minutes until lightly golden. Gradually whisk in the reserved cooking liquid, stirring constantly for 3-4 minutes until thickened and smooth.
08 - Place a slice of bread on each serving plate. Top with a generous mound of mashed potatoes, followed by a portion of shredded beef. Ladle hot gravy generously over the entire assembly.
09 - Serve immediately while hot.