In-N-Out Double Double - Animal Style (Copycat)

ground beef ((Kenji recommends 60/40, I used 80/20 and it was perfect))

butter

large onion (, finely chopped (about 1 1/2 cups))

Kosher salt

plus 2 teaspoons mayonnaise

ketchup

sweet pickle relish

sugar

distilled white vinegar

soft Hamburger buns (, (Kenji recommends Arnold, I used local bakery buns))

Freshly ground black pepper

dill pickles chips

quarter-inch-thick slices ripe tomato

leaves fresh iceberg lettuce (, white core section removed, torn to bun-sized pieces)

yellow mustard

slices deli-cut American Cheese ((I bought this from my grocer's deli case))