Preheat a panini grill or a George Foreman grill and lightly grease it with non-stick spray (or preheat an oven to 350°F).
Slice the bread open. Spread mustard on both sides.
Layer cheese, pickles, and pastrami on the bread.
Close the sandwich, place on a panini grill (or George Foreman grill) and press down. Cook until the cheese is melted and bread is crispy. (If using an oven, place the sandwich on a baking sheet and cook until the cheese is melted.)