Place the finely diced white onion in a small bowl and cover with cold water. Let it soak for 5 minutes while you prep the rest of the ingredients, then drain well. This step is the secret to that authentic McDonald's onion flavor — the cold soak mellows the sharp raw bite and perfectly mimics the rehydrated onions used at the restaurant.

Divide the ground beef into 4 equal balls, about 2 ounces each. Keep them loose and barely shaped — do not pack them tight. Loose meat spreads better when smashed and creates those signature crispy edges. Hold off on seasoning until the patties hit the pan.

Heat a cast-iron skillet or flat griddle over medium-high heat for about 4 minutes, until very hot and almost smoking. Add the neutral oil and swirl to coat the surface evenly.

Place the beef balls on the hot pan with plenty of space between them. Immediately smash each one flat with a sturdy spatula until paper-thin, about 4 inches wide. Season the tops generously with kosher salt and black pepper.

Cook the patties undisturbed for 3 minutes, until a deep brown crust forms and the edges turn crispy and lacy. Scrape under each patty with a thin spatula, flip, and immediately top each one with a slice of American cheese. Cook for 1 more minute to melt the cheese into the meat.

Place the hamburger buns cut-side down on an empty spot in the pan. Add the water and cover with a lid for 30 seconds. The steam softens the buns until pillowy — this is the McDonald's bun secret most home cooks skip.

On each bottom bun, layer in this exact order: yellow mustard, ketchup, dill pickle chips, and a sprinkle of the drained diced onions. Stack two cheese-topped patties on each burger, then crown with the top bun. Press gently to compress everything together and serve immediately.