Use a serrated knife to carefully slice the croissants in half.
Brush the cut sides of the croissants with melted butter and lightly toast them in a dry skillet over medium heat until golden brown, then set aside.
Divide the ground beef into four equal portions and roll them into loose balls without packing them too tightly.
Heat a cast-iron skillet over high heat until smoking hot.
Place the beef balls in the pan and immediately press them down firmly with a sturdy spatula to smash them thin.
Season the patties generously with salt and pepper.
Sear the patties undisturbed for 2-3 minutes until a dark, crispy crust forms.
Flip the patties, top each with a slice of cheese, and cook for another 1-2 minutes until the cheese is melted and the beef is cooked through.
Assemble the burger by placing the cheesy patty on the bottom croissant half, topping with crispy turkey bacon and caramelized onions, and drizzling with your chosen sauce before closing with the top bun.