Blend first 6 ingredients. Form into 8 thin patties and place each on sheets of waxed paper. Stir together mustard, onion, relish and cheese. Place a spoonful of this mixture in the center of four patties. Smooth out leaving a rim of meat at the edge. Top with remaining patties, pressing edges to seal. Brush with melted butter. Grill burgers or fry at medium until browned.
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