01 - Rub the beef chuck roast thoroughly with kosher salt and black pepper on all sides to ensure even seasoning.
02 - Arrange sliced onions and minced garlic in an even layer at the bottom of the slow cooker. Place the seasoned beef roast directly on top of the vegetable bed.
03 - Pour the beef broth and Worcestershire sauce over the beef and vegetables, ensuring the roast is partially submerged.
04 - Cover the slow cooker and cook on low heat for 8 hours. The beef is ready when it pulls apart easily with minimal resistance.
05 - Forty minutes before beef finishes, place cubed potatoes in a large pot and cover with cold water. Add a generous pinch of salt and bring to a boil. Cook until fork-tender, approximately 15-20 minutes. Drain thoroughly and mash with butter, milk, salt, and pepper until smooth and creamy.
06 - Transfer the cooked beef from the slow cooker 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 butter in a saucepan over medium heat. Whisk in flour and cook for 1-2 minutes until mixture turns lightly golden. Gradually whisk in the reserved cooking liquid, continuing to stir until the gravy thickens and achieves smooth consistency, about 3-4 minutes.
08 - Place one 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 beef remains tender.