How to Make It
To prepare pickles, combine vinegar, dill, salt, and cucumber in a large zip-top plastic bag or jar; seal. Turn bag until ingredients are well combined. Refrigerate at least 1 hour, turning bag or shaking jar occasionally.
To prepare burgers, place cremini and button mushrooms in the bowl of a food processor; pulse 4 to 5 times or until coarsely chopped. Heat a large skillet over medium-high heat. Add oil to pan; swirl to coat. Add onion and garlic; sauté 3 minutes. Add mushrooms, 1/4 teaspoon salt, and 1/8 teaspoon pepper to pan; cook 7 to 10 minutes or until liquid evaporates, stirring occasionally. Remove pan from heat; cool mushroom mixture completely.
Place beef in a bowl. Add mushroom mixture to beef; stir just until combined. Refrigerate 10 minutes. Shape beef mixture into 4 (1/2-inch-thick) patties. Sprinkle evenly with remaining 1/4 teaspoon salt and remaining 1/8 teaspoon pepper.
Heat a large cast-iron skillet over medium-high heat. Coat pan with cooking spray. Add patties to pan; cook 2 minutes on each side or until desired degree of doneness.
Combine mayonnaise, ketchup, and mustard in a small bowl, stirring with a whisk. Spread mayonnaise mixture evenly over bottom and top halves of buns. Place 4 pickle slices on bottom half of each bun (reserve remaining pickles for another use). Top evenly with patties, lettuce, tomato, onion, and top halves of buns.
Amazing Recipe Makeovers, Oxmoor House, copyright 2016.