Preheat the broiler. Arrange the pretzel rolls on a baking sheet. Broil for one minute, until the bread is slightly dry and crispy - two minutes if your broiler isn't very strong. Then, flip the bread and broil on the second side for the same amount of time.

Meanwhile, combine the shrimp with the mustard and set aside.

To assemble, spread a bit of shrimp mixture on the cut side of the pretzel bread, then top with 2 pieces (quarters) of cheese. Or, layer cheese then shrimp. Broil for another 5 minutes or so until cheese is bubbly and shrimp is just barely cooked through.

While hot, sprinkle with coarse salt. Serve with extra mustard and sweet relish, if desired.