Heat a large skillet over medium-high heat and cook the burger patties for about 7 minutes on each side, or until internal temperature has reached 165F.
When the patties are almost finished, top each one with 1 teaspoon of yellow mustard and a slice of American cheese. Cover with a lid to help the cheese melt and then add 1 tbsp. caramelized onions on top of 3 of the patties. Stack one patty on top of each caramelized onion topped patty, sandwiching the onions in the middle.
Layer each burger in the following order from the bottom up: bun, fry sauce, 4 pickles, 1 slice of tomato, iceberg lettuce, double patty, top bun.
Enjoy immediately... grab a napkin.
Recipe by Slice of Jess at https://sliceofjess.com/making-in-n-out-secret-menu-at-home-animal-style/