01 - Rub the beef chuck roast thoroughly with kosher salt and black pepper on all sides to ensure even seasoning.
02 - Arrange the sliced yellow onions and minced garlic in an even layer at the bottom of the slow cooker insert.
03 - Place the seasoned beef roast on top of the vegetables. Pour in the beef broth and Worcestershire sauce.
04 - Cover and cook on low heat for 8 hours until the beef is fork-tender and easily shreds apart.
05 - About 40 minutes before beef finishes, place cubed potatoes in a large pot with cold water and salt. Bring to a boil and cook 15-20 minutes until tender. Drain thoroughly, then mash with 4 tbsp butter, milk, salt, and pepper until smooth and creamy.
06 - Transfer the cooked beef to a cutting board. Using two forks, shred the meat into bite-sized pieces. Strain the cooking liquid and reserve 1 cup for gravy preparation.
07 - Melt 2 tbsp butter in a saucepan over medium heat. Whisk in flour and cook 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 the gravy is hot and the bread retains some texture from the toppings.