Make the Special Sauce:
In a small bowl, combine the mayonnaise, ketchup, mustard, relish, vinegar, garlic powder, salt, and pepper. Mix everything together until it's smooth and well blended. Taste and adjust seasonings if you like it a little more tangy or sweet. Set this sauce aside.

Caramelize the Onions:
Heat a bit of oil or butter in a pan over medium heat. Add diced onion and cook, stirring occasionally, for about 10-15 minutes, or until the onions are soft, golden brown, and caramelized. Add sugar, Worcestershire salt and pepper. Continue to cook for another 30 minutes. The longer you cook them, the sweeter and more flavorful they’ll get. Set them aside once they’re perfectly golden and tender.

Cook the Patties:
Preheat a cast-iron skillet or griddle to medium-high heat. Form your ground beef into two evenly sized patties, slightly larger than the buns to account for shrinkage. Season each side with salt and pepper. Once the skillet is hot, add the patties and press down cook for about 3-4 minutes on the first side until crispy and browned. Drizzle some yellow mustard on patty before flipping. Flip the patties, and place a slice of American cheese on each one. Cook for another 2-3 minutes until the cheese is melted and the patties are cooked to your preferred doneness.

Toast the Buns:
While the patties are cooking, slice your burger buns in half and toast them lightly in a separate pan or on the grill until they’re golden brown and crisp.

Assemble the Burger:
Spread a generous amount of your homemade special sauce on the bottom half of the toasted bun. Place a patty on top of it and add a second patty, stacking it right on top for that double-patty goodness. Pile on the caramelized grilled onions and a few dill pickle slices for that perfect tang. Add lettuce and tomato. Spread a little more special sauce on the top bun, then place it gently on top of your burger stack. Press it down slightly to make sure everything stays put.