Divide the ground beef into two equal portions and gently form into loose balls, avoiding overworking so the meat stays tender.
Preheat your skillet over high heat until just smoking, then add a small amount of oil to coat the surface.
Place each beef ball onto the hot skillet and immediately press down firmly with a spatula until each patty is very thin, about 1/4 inch thick.
Sprinkle a generous pinch of salt over each patty right after smashing; listen for the sizzle as the crust begins to form.
Cook the patties for about 2-3 minutes until the edges are deep golden brown and crispy, then flip carefully with a spatula.
Immediately top each patty with a slice of cheese, pressing it gently into the hot meat so it melts quickly.
While the cheese melts, toast the buns in a separate pan or in butter until golden and crispy, about 1-2 minutes.
Remove the cheese-topped patties from the skillet once the cheese is melted and the crust is crispy; let them rest for a minute.
Assemble the burger by spreading condiments on the toasted buns, then layering pickles, sliced onions, and the crispy beef patty with melted cheese.
Top with the other half of the bun, give it a gentle press, and serve immediately while hot and crispy.
