Make the sauce: Mix the onion, both mustards, the mayonnaise, paprika, garlic powder and salt to taste in a bowl. Set the sauce aside.
Make the chips: Heat 1/2 inch vegetable oil in a large skillet over medium heat. Gently pat the pickle slices dry with a paper towel.
Put the pretzels in a large resealable plastic bag and crush into coarse crumbs using a rolling pin or heavy pan; transfer to a plate. Whisk the eggs in a shallow bowl and put the flour in another bowl. Dredge the pickle slices in the flour, shaking off any excess; dip in the beaten eggs, then dip in the pretzel crumbs, turning to coat. Working in batches, fry the pickles until golden and crisp, about 1 minute per side. Drain on paper towels and sprinkle with celery salt, if desired. Serve warm with the mustard sauce.