Combine the garlic powder, black pepper and 1/4 teaspoon kosher salt in a small bowl and set aside.
Combine the onions and 2 teaspoons kosher salt in a bowl and set aside for 10 minutes. Strain and reserve the juices.
Line a half sheet pan with parchment paper and place the ground chuck in the middle of the pan. Cover the meat with a large sheet of plastic wrap and roll out the meat with a small rolling pin into a large rectangle that covers the surface of the pan; it will be very thin. Remove the plastic wrap. Add a thin layer of yellow mustard and season the meat with the spice mixture.
Pick up the edge of the parchment and fold the meat in half, widthwise. Roll a couple of times just to seal the two layers. Refrigerate for 10 minutes then cut into 8 even squares with a pizza cutter.
Preheat a griddle pan over medium heat.
Spread the onions out on the griddle in an even layer in a rectangular shape. Put the patties on top of the onions. Resist the urge to press them with your spatula. Squeezing burger patties is the culinary equivalent of picking at a scab, and you remember what your mom told you about that. Pour over some of the remaining onion juice to create steam. Top each patty with a slice of cheese and the bottom half of a bun. Cover the griddle with foil (but do not seal) and reduce the heat to low. Steam the burgers until cooked, 3 to 5 minutes.
Construct burgers thusly: Add pickles and mustard to each bun top. Using a large spatula, pick up a burger-bottom bun stack and place it on the top half of a bun. Repeat with the remaining burger stacks and bun tops until there's nothing left to build.