To assemble the sliders: Turn on the broiler, or begin heating your toaster over. Slice the pretzel rolls in half and slather mayonnaise and mustard on each side. Layer 3 slices of pastrami on the bottom half and a slice of provolone on top of the pastrami. Place both halves of the rolls, side by side on a baking sheet. Put under the broiler for 2 minutes, or until cheese is melted. Add a few onions on top of the cheese, and place top half of the roll on the completed sandwich. Serve immediately!