Whisk together all Secret Burger Sauce ingredients in a small bowl. In a separate bowl, mix the Herb Butter Glaze ingredients (melted butter, garlic, parsley, salt). Set both aside.

Slice croissants in half, brush the cut sides with plain melted butter, and toast on a hot skillet until golden brown. Set aside.

Divide cold ground beef into 8 loose balls (about 3 oz each). Do not compress the meat.

Heat a cast-iron skillet or griddle until smoking hot. Place a meatball on the surface, top with a small piece of parchment paper, and smash firmly with a heavy spatula until very thin. Remove paper and season generously with salt and pepper.

Cook for 2-3 minutes until the edges are dark, crispy, and lacy. Scrape firmly under the patty to keep the crust intact and flip.

Immediately place a slice of American cheese on each patty. Cook for 1 more minute until cheese is melted.

Assemble: Spread burger sauce on the bottom croissant half. Add lettuce and tomato. Stack 1 or 2 cheesy smash patties on top, followed by caramelized onions.

Spread sauce on the top croissant bun, place it on the burger, and brush the top generously with the Herb Butter Glaze. Serve immediately.