This dish is part of our pub fare menu.
Mix spices, sugar, and salt in a bowl. Stir in molasses to form a dry paste; rub all over beef. Place meat in a nonreactive container or resealable bag. Let marinate 4 to 7 days in the refrigerator, turning and rubbing beef once each day.
Place beef and stout in a wide (6- to 8-quart) pot and add water to just cover beef. Bring to a simmer; cover and cook until tender but not falling apart, about 3 hours (30 minutes more if using chuck). Remove from heat, but let beef sit in pot for 2 hours. When cool, remove beef and chill in refrigerator at least 2 hours.
Slice meat and assemble sandwiches with bread; if desired, add horseradish to taste. Serve with coleslaw and pickles.
Chef Armstrong recommends pairing this dish with Guinness.