Preheat your Blackstone griddle to medium–low heat.
While it heats, chop the onion and divide the beef mince into eight 100 g balls.
Using greaseproof paper, gently press each ball into a flat patty slightly larger than the slices of cheese.
Place two slices of American cheese on four of the patties, then top each with another patty. Press the edges together gently to seal the cheese inside.
Lightly oil the Blackstone using your preferred cooking oil.
Add the chopped onions to the griddle and cook until they begin to soften.
Place the burger patties on the griddle and season with salt to taste.
Cook the burgers over medium–low heat, allowing them to develop a slow sear while keeping the cheese sealed inside.
Once the underside has developed a good color, carefully flip the burgers.
Continue cooking until they are cooked through, about 6–8 minutes total. Increase the heat briefly at the end if you want a stronger sear.
Remove the burgers from the griddle, place them on buns with pickles and onions, stack if desired, and serve. Enjoy!