Prepare the Onions:
Slice onions very thin using a mandolin. Toss with kosher salt and let sit for 20 minutes to draw out moisture. Squeeze out excess liquid and set aside.

Form the Patties:
Divide ground beef into equal portions (about 1/4 pound each) and gently roll into balls. Place in the refrigerator for 15 minutes to firm up.

Heat the Griddle:
Preheat a griddle or large cast-iron skillet over high heat for about 10 minutes. Lightly oil if needed.

Make the Sauce:
In a bowl, whisk together mayo, ketchup, mustard, sweet relish, hot sauce, pepper, and salt until smooth. Set aside.

Smash the Burgers:
Place beef balls onto the hot griddle. Immediately top each with a generous handful of salted onions.
Smash firmly with a spatula until very thin. Season with salt.

Cook & Flip:
Cook for about 4 minutes until deeply browned and crispy. Flip, season lightly, and cook another 2 minutes.
Add American cheese and allow it to melt. Place buns on the griddle to toast lightly.

Steam the Buns (Optional but Recommended):
Stack the top bun over the patty and place the bottom bun on top briefly. Cover with a lid for a minute to steam and soften.

Assemble:
Spread burger sauce on the bottom bun, add pickles, then top with one or two patties. Finish with the top bun.

Serve:
Serve immediately while hot and crispy.