Divide the beef into 8 equal portions. Roll the beef into balls, transfer to a plate, and cover with plastic. Refrigerate for 30 minutes (and up to 24 hours).
Meanwhile, to make the burger sauce, combine the mayonnaise and mustard and mix well.
Preheat a griddle or heavy skillet to medium-high heat.
Place the burger balls on the hot pan (space them out so they aren’t touching). Working quickly, place a piece of parchment paper over the beef and press down with a burger press or heavy spatula, creating thin patties.
Once smashed, remove the parchment paper and season the patties with the garlic powder, salt, pepper, and ½ teaspoon of the burger sauce.
When the bottom of the burger is browned and crisp, flip and cook for 30 to 60 seconds, until the second side is browned and crisp.
Top half of the patties with the cheese slices.
Top the cheese with the second patty and cook for 1 minute.
Transfer the patties to a plate.
If desired, toast the buns on the same hot griddle.
To assemble the burgers, place a small amount of burger sauce on the bottom bun. Top with the lettuce, tomato, pickles, and red onion.
Arrange the patties over top, place the top bun on top and serve.