Mix melted butter with garlic and parsley, then brush over croissants and lightly toast them.
Heat a pan on high heat, place beef portions and smash them flat. Season with salt and pepper.
Cook until crispy on one side, flip, then add cheddar cheese and let it melt.
Mix all burger sauce ingredients in a small bowl.
Slice croissants in half and spread sauce on both sides.
Layer with lettuce, tomato, and the cheesy smashed beef patty.
Close the croissant and serve immediately