Recreate the iconic In-N-Out Double Double burger at home, complete with special sauce, grilled onions, and perfectly seared patties with melted American cheese on toasted buns.
1. Cut four pieces of parchment paper in 4.5-inch squares. Mix the ground beef with 1/2 tsp kosher salt and 1/4 tsp pepper, then form into four equal balls. Press the balls onto the parchment paper into four thin 4.5-inch patties (they will shrink as they cook) and set aside in the fridge to firm up.
2. Mix together the ingredients for the sauce (Mayo, Ketchup, Sweet Pickle Relish, White Vinegar, Yellow Mustard, and a tiny pinch of salt) and set aside.
3. In a large pan over medium heat, melt 2 Tbsp butter and then add the diced onions. Sprinkle with salt and pepper and sauté until tender and deep golden brown (10-12 minutes). Remove from the pan.
4. Spread a thin layer of softened butter on the cut side of the burger buns and toast cut side down in the pan over medium heat for 1-2 minutes until deep golden brown. Remove from the pan.
5. Add 1 tsp oil to the pan along with the burger patties. Sear patties for 1-2 minutes on the first side (until browned), flip, and add a slice of American cheese on top of each patty. Cook 1 more minute then stack patties on top of each other (so that there are two patties and two slices of cheese per burger).
6. On the bottom bun, add special sauce, tomato, lettuce, the burger patties with cheese, and grilled onions (in that order). Top with the other side of the bun and enjoy!
