Slice all of the onions using a mandolin as thin as possible. Toss them in a bowl with kosher salt and let them sit for about 20 minutes. Squeeze out any excess water to concentrate their flavor.

Roll the ground beef into equally portioned balls, about 1/4 pound each, and let them rest in the refrigerator for 15 minutes.

In a bowl, whisk together mayo, ketchup, mustard, sweet relish, hot sauce, pepper, and kosher salt until nicely blended to make the burger sauce.

Heat a griddle on high for about 10 minutes, adding a bit of oil if necessary.

Place the burger balls on the griddle, top each with a handful of onions, and smash them down into thin patties. Season with salt.

Cook for about 4 minutes, then flip and season again. After an additional 2 minutes, place slices of American cheese on top of each patty to melt.

Cover with the top and bottom buns to create some steam.

Remove the bun stack and add a generous amount of burger sauce and pickles to the bottom bun. Layer on one or two patties and finish with the top bun.