Steps

prep any sauce you want to use like Shack Sauce

sear the sweet onion in a pan on medium-high heat with butter in a pan (non non-stick

set the onion aside and toast burger buns

turn up the heat to high and don't add additional oil (note: The fat content of the beef is enough for searing and preventing sticking. Adding oil can actually hinder the desired crust formation and result in a less crispy burger)

loosely pack the ground beef into 2 balls. Don't pack them too much because we want craggy bits

add one of the balls to the pan and using a spatula, smash the patty into the pan and season the top with salt and pepper

scrape the burger and flip once the bottom is crisp and it's about 80% cooked. Add cheese and cook until done ~15 seconds

repeat with the other burger

assemble with bun -> half the sauce -> pickle slices -> first patty with cheese -> seared onions -> 2nd patty with cheese -> red onion -> remaining sauce -> bun