Try this delicious sandwich from Cathal Armstrong, chef at Restaurant Eve in Arlington, Va. It's a bit of a project, but the beef takes on tremendous amounts of flavor and is well worth the wait. With a 4-pound roast, there will be plenty of sandwiches to go around.
In a bowl, mix all of the spices together. Add the molasses and rub the mixture all over the beef. Place in a nonreactive container in the refrigerator for 7 days. Every day, turn and rub the beef.
Place the beef in a large pot and add the Guinness and water to cover. Bring to a simmer and cook until tender but not falling apart, about 3 hours. Remove from heat and let the roast cool in the stock.
When cool, remove the beef and refrigerate until cold before serving. Serve with wheat or pumpernickel bread and pickles. Horseradish can be added to taste.