Preheat the oven to 450°F [230°C], with the convection option on, if you have it.
In a medium bowl, combine the tuna, mayon¬naise, apple, celery, parsley, shallot, garlic, salt, and pepper and stir until well mixed. Set aside.
Place the pretzel bun halves on a baking sheet, cut-side up. Spread 1 tsp mustard on each bun half. Layer two tomato slices, two or three pickle chips, one-fourth of the tuna mixture, and one slice of the Gouda on top of each bun half.
Bake the open-faced sandwiches for about 3 minutes, or until the cheese is melted.
Cut each sandwich in half, if desired, and serve immediately.