01 - Rub the beef chuck roast thoroughly with kosher salt and black pepper on all sides.
02 - Arrange sliced onions and minced garlic in the bottom of the slow cooker. Place the seasoned beef roast on top of the vegetables.
03 - Pour the beef broth and Worcestershire sauce over the beef and vegetables.
04 - Cover and cook on low heat for 8 hours, or until the beef is very tender and shreds easily with a fork.
05 - About 40 minutes before the 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 for 15-20 minutes until fork-tender. Drain and mash with butter, milk, salt, and pepper until smooth and creamy.
06 - Remove the cooked beef from the slow cooker and shred using two forks. Strain the cooking liquid and reserve 1 cup for making the gravy.
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 until thickened and smooth, about 3-4 minutes.
08 - Place a slice of bread on each serving plate. Top with a mound of mashed potatoes, followed by a generous portion of shredded beef. Ladle hot gravy over the entire dish.
09 - Serve the Beef Manhattan hot while the gravy is still warm and the bread retains some texture.