In a large bowl or ziploc, combine 4 large chicken thighs and 1 1/2 cups pickle brine. Marinate for 30 minutes at room temperature, or a maximum of 4 hours in the fridge. Don’t marinate for longer than that – the acid in the brine will make the chicken a funky texture.
In a separate large bowl, make the special sauce. Whisk together 1/3 cup mayonnaise, 1/4 cup chopped pickles, 2 teaspoons Dijon mustard, 2 teaspoons Worcestershire, 1 teaspoon pickle brine, 1/4 teaspoon garlic powder, and a big pinch of kosher salt.
Preheat a grill or grill pan to medium-high and spray with cooking spray. Grill the chicken for 4 to 5 minutes per side, or until cooked through. While the chicken rests, quickly toast the inside of 4 buns for about 30 seconds.
Slather the bottoms of the buns with special sauce, and pile iceberg on top. Then the chicken, some tomato slices if they’re in season, some pickles, more special sauce on the top bun, and close the sandwich. Devour immediately!