Caramelize the onions

• Add the butter to a skillet, then add the chopped onion and kosher salt.
• Cook over medium-low heat, stirring continuously, until the onions are fully caramelized.

Make the burger spread

• In a small bowl, combine the mayonnaise, ketchup, sweet pickle relish, sugar and distilled white vinegar.
• Whisk until smooth and well combined.

Form the patties

• Divide the ground beef into four equal patties of about 57 g each.

Smash and cook the patties

• Heat the griddle on high and add the remaining butter.
• Add the beef patties and smash them down flat.
• Cook the patties, and meanwhile place the buns face down on the griddle to lightly toast.

Mustard grill and top the patties

• Spread mustard on top of each patty and flip it.
• Season with salt and black pepper, then top each patty with a slice of American cheese and a spoonful of caramelized onions.
• Cook for 1 minute after flipping.

Assemble the burgers

• Spread the burger sauce on both sides of the toasted buns.
• On each bottom bun, layer the pickles, then the tomato slices, then the lettuce.
• Stack two patties on top of the lettuce and close with the top bun.