To make a hot pastrami sandwich, caramelize onions and warm pastrami. Layer the meat and onions with cheese and mustard on toasted or grilled bread. There are many variations on the hot pastrami sandwich, some use beer while others use mayonnaise.
FoodNetwork.com offers a sandwich recipe that calls for a little extra work in the form of simmering the pastrami in a mixture of beer and mustard. The sandwich itself consists of the meat, grilled onions and rye bread. This version does not use cheese.
Tillamook.com's variation features a butter and mayonnaise spread. The recipe recommends heating the pastrami on a griddle before assembling the sandwich. The cook layers the meat, black pepper white cheddar cheese, Muenster cheese, caramelized onions and pickles between prepared bread. Grill the entire sandwich to melt the cheese slowly.
HalfBakedHarvest.com recommends slowly caramelizing onions with sugar, then adding beer. The recipe directs the cook to simmer the onions until the beer thickens, and then add Worcestershire sauce and mustard. This recipe also suggests heating the pastrami before building the sandwich. The sandwich consists of the meat, onions, Jarlsberg cheese, and pumpernickel or rye bread. The recipe recommends cooking the sandwich in butter on both sides until it is toasted.