The In-N-Out "Double Double" burger — served off-the-menu "Animal Style" — is a West Coast burger legend. To put the craze into perspective, people have gone as far as to freeze-dry the In-N-Out classics and ship them cross-country. Whether it’s due to the secret sauce (that some believe is really just Thousand Island dressing), the caramelized onions, or the fact that the burger patties are never frozen, it’s clear that this is an iconic burger that’s worth making at home.
Mix the ketchup, mayonnaise, and sweet pickle relish together until the dressing reaches the desired pink color.
Heat 2 tablespoons of the oil in a skillet over medium heat. Cook the onions on low, stirring frequently, until caramelized. When caramelized, deglaze the pan with 2 tablespoons of water, and then repeat the caramelization process until the onions have the consistency of jam.
In a skillet over medium-high heat, sear both the meat patties on 1 side, then spread mustard on the other. Flip the patties so that the mustard side cooks on the grill.
Put the 2 slices of cheese on the patties long enough for them to melt. Lightly toast both sides of the bun on the grill while you wait for the cheese to melt.
Take the burger buns and patties off the grill, and build your burger. Spread the caramelized onions between the cheesy patties, add the lettuce, tomatoes, and pickles, and then spread the Thousand Island sauce on the lower bun.