In a bowl, combine pickles, onion and mustard until well incorporated. Hold refrigerated for service.
Set grill to medium high. For each serving, season two slider patties each with a 1/4 teaspoon of hamburger seasoning. Grill until to desired doneness, just before finish top each slider with cheese to melt. Remove from grill.
Place grilled patties on the heels of two pretzel buns. Top each with Mustard Onion Relish, a tomato slice, and 2 tablespoons of lettuce. Serve hot.
This relish also serves well on hot dogs and grilled sausages.
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