Croissant Smashburger with Sweet & Tangy Sauce

1. Prepare the burger sauce: In a small bowl, combine the mayonnaise, ketchup, sweet pickle relish, sugar, and distilled white vinegar. Mix thoroughly until well combined and set aside.
2. Cook the wagyu beef patties: Form the wagyu beef into three thin patties suitable for smashburgers (e.g., 2-3 oz each). Heat a griddle or cast-iron skillet over medium-high heat. Place the patties on the hot surface and smash them down with a sturdy spatula. Season with salt (if not relying solely on the flakey salt later). Cook for 1-2 minutes per side until browned and cooked through. During the last minute of cooking, place a slice of American cheese on each patty to melt.
3. Cook the bacon: Cook the bacon to your desired crispness, either in a skillet or oven.
4. Prepare caramelized onions if not already done.
5. Toast the croissant until lightly golden and warmed through.
6. Assemble the burger: Start with the bottom half of the toasted croissant. Layer with fresh lettuce, a generous dollop of the prepared burger sauce, then stack the three cheesy smashburger patties. Add a layer of caramelized onions, followed by the crispy bacon. Sprinkle with black flakey salt to taste. Finish with more burger sauce and the top half of the croissant.