01 - Rub the beef chuck roast generously with kosher salt and black pepper, coating all sides evenly.
02 - Arrange the sliced onions and minced garlic in the bottom of the slow cooker, then place the seasoned beef roast on top.
03 - Pour the beef broth and Worcestershire sauce over the beef and vegetables.
04 - Cover and cook on low heat for 8 hours until the beef becomes very tender and shreds easily with a fork.
05 - Place cubed potatoes in a large pot, cover with cold water, and add a generous pinch of salt. Bring to a boil and cook for 15-20 minutes until fork-tender, then drain thoroughly.
06 - Mash the drained potatoes with butter, milk, salt, and pepper until smooth and creamy, about 40 minutes before the beef finishes cooking.
07 - Remove the cooked beef from the slow cooker and shred using two forks. Strain and reserve 1 cup of the cooking liquid for making gravy.
08 - Melt butter in a saucepan over medium heat, then whisk in flour and cook for 1-2 minutes until lightly golden and fragrant.
09 - Gradually whisk the reserved cooking liquid into the roux, stirring constantly for 3-4 minutes until thickened and smooth.
10 - Place a slice of bread on each serving plate, top with mashed potatoes, then add a generous portion of shredded beef. Ladle hot gravy over the entire dish and serve immediately.