Set 2 burners to high and 2 burners to medium on a 36-inch Blackstone® Griddle Cooking Station and preheat for 10 minutes.

Meanwhile, stir the ketchup, mayonnaise, mustard and chopped pickle in a small bowl and set aside. Stir the garlic powder, onion powder, 2 teaspoons salt and ¼ teaspoon pepper in a separate small bowl. Set aside.

Loosely form the beef into 8 balls; set aside. Working in batches, lightly brush butter onto the medium-heat side of the griddle and toast the buns cut-side down until lightly browned, about 5 minutes. Flip and keep warm while you cook the burgers.

Brush butter onto the high-heat side of the griddle and add the onions. Cook, tossing with a metal spatula occasionally, until starting to brown slightly, about 3 minutes. Divide the onions into 4 equal piles, spacing them a couple inches apart. Top each pile with 1 ball of beef. Smash each ball with the back of a spatula or a burger press until the patty is about ¼ inch thick. Repeat with the remining 4 beef balls directly on the griddle. Season the patties with half the seasoning mixture and continue cooking, undisturbed, until well browned, 1 ½ to 2 minutes. Flip the patties, sprinkle with the remaining seasoning mixture and top each plain patty with 1 cheese slice. Continue cooking until the cheese is melted around the edges and the bottoms of the patties are browned, about 1 more minute.

Spread the ketchup mixture on the bun tops. Fill each bun with 1 onion patty, 1 plain patty, 1 tomato slice, 1 lettuce leaf and some of the onions, if any, that were left on the griddle.