Divide the 1/2 lb ground beef into a single patty: gently form into a loose ball and flatten to a round patty about 4 inches in diameter and roughly 3/4 inch thick. Do not overwork the meat; handle lightly so the patty stays tender. Make a shallow 1/2-inch-wide indentation in the center with your thumb to help it cook flat.

Season

Season both sides of the patty evenly with the kosher salt and freshly ground black pepper just before cooking.

Cook Patties

Heat a cast-iron skillet or heavy griddle over medium-high heat until very hot, about 4 minutes. Add the neutral oil and spread to coat. Place the patty on the hot surface; you should hear a sizzle. Cook undisturbed for 3 to 4 minutes until a deep brown crust forms on the bottom.

Flip the patty and reduce heat to medium. Cook for another 3 to 4 minutes for medium (internal temperature about 145–155°F / 63–68°C). If using cheese, place the slice on the patty during the last 1 minute of cooking and cover the pan with a lid to melt the cheese.

Remove the patty to a plate and let rest 2 minutes to redistribute juices.

Toast Bun & Assemble

While the patty rests, lightly butter the cut sides of the bun with the unsalted butter. Return skillet to medium heat and toast the buttered bun cut-side down 30–45 seconds until golden and crisp. Remove from skillet.

Spread the mayonnaise on the bottom bun and the mustard on the top bun (or vice versa to preference).

Place the rested cheeseburger patty on the bottom bun. Top with lettuce leaf, tomato slices, pickle slices, and onion if using. Cap with the top bun.

Serve immediately while hot.