Copycat In-N-Out Double-Double Animal-Style Burger

In a small bowl, whisk together the mayonnaise, ketchup, relish, sugar, and vinegar until well combined. Set aside the special sauce. (Prep time: 5 minutes)

In a large nonstick skillet, melt the butter with 1 tablespoon of canola oil over medium heat. Add the finely chopped yellow onions and season with ¼ teaspoon of salt. Cook, stirring occasionally, until the onions become tender and develop a rich, golden-brown color, about 20 minutes. If the skillet starts to dry out, add water, 1 tablespoon at a time, stirring often until the onions are very soft, deeply browned, and jammy, approximately 10 to 15 minutes more. Be careful not to burn them. Remove from heat and set aside. (Cook time: 30-35 minutes)

Divide the ground beef into 8 equal portions and gently shape them into patties, about 3/8-inch-thick and 4 inches in diameter. Season the patties with black pepper and the remaining 1/2 teaspoon of salt. (Prep time: 10 minutes)

Heat 1 teaspoon of canola oil in a large cast-iron skillet over medium-high heat. Working in batches as needed, carefully place the patties into the hot skillet. Spread 2 teaspoons of mustard evenly over the top of each patty. Cook until the bottoms are nicely browned, about 2 minutes per side. Flip the patties and top each with 2 slices of American cheese and a generous portion of caramelized onions. Cover the skillet and continue cooking for about 30 seconds, or until the cheese is perfectly melted and gooey. Use a spatula to stack the patties in pairs, creating 4 delicious stacks. (Cook time: 10-15 minutes)

Spread a generous amount of the special sauce onto the bottom half of each toasted hamburger bun. Layer each bun with 4 dill pickle chips, a slice of fresh tomato, a piece of crisp lettuce, and one of the mouthwatering patty stacks. Top with the other half of the bun and serve immediately. (Assembly time: 5 minutes)