Thinly slice onions using a mandolin or sharp knife.
Form ground beef into quarter-pound pucks, season with salt and pepper.
Preheat Ninja Woodfire Grill to Hi with charcoal pellets.
Place beef pucks on the grill, top with onions, and smash to flatten.
Cook for 4-5 minutes, allowing onions to caramelize.
Flip and cook for an addition 1-2 minutes.
Place 1 slice of cheese on the patty if desired and then add the top bun just side down. Place the bottom bun on top of the top bun cut side up.
Pour a little bit of water on the griddle and then cover with a large metal bowl to steam for 1-2 minutes.
Take the bowl off being careful as it will be very hot. Remove the burgers from the grill.
Assemble burger with optional mustard and pickles, and serve immediately.