Place beef in pressure cooker, in smaller chunks if necessary. Add sliced onion, seasonings, and beef broth. Cover and secure lid. Cook in pressure cooker for 60 minutes. Allow pressure to release naturally for 15-20 minutes before carefully venting remaining steam. (Alternately, you can cook in a slow cooker on low for eight hours.)
Carefully transfer cooked beef to a cutting board and slice thinly or shred into pieces. Return to pot with juices and stir to combine.
When ready to build sandwiches, butter and toast the buns over a grill.
Arrange bottom buns in a foil baking pan; top with sliced or shredded beef, 1 slice cheese, and top bun. Cover pan with lid or aluminum foil and set over warm grill to allow cheese to melt.
Enjoy sandwiches warm with desired condiments such as pickles or horseradish-mayo.