To make the secret sauce, add mayonnaise, mustard, pickle juice, smoked paprika, garlic powder, onion powder, and hot sauce (if using) to a medium bowl and stir to combine. Set aside.

Divide ground beef into 4 equal, loosely packed, ball-shaped portions (they are not patties—yet). Lightly oil onion slices, keeping rings as intact as possible. Slather butter on buns.

Heat a Carbon Steel Griddle on the grill or stove top over medium-high until very hot, 3–5 minutes. Lightly drizzle the surface of the griddle with oil and use a heat safe brush or paper towel to coat evenly.

Add sliced onion and cook until charred on both sides, 1–2 minutes per side. Transfer to a Sheet Pan and set aside.

Working in batches if needed, place beef portions on griddle and smash flat with an oiled Grill Press to form thin patties, between ¼” and ½”-thick. It’s ok if your patties are not perfectly round, the uneven edges will become browned and crispy. Season generously with salt, cover the grill, and cook until edges are brown and juices start to accumulate on top, about 2 minutes.

Flip patties with a large heatproof spatula (do not smash again), season lightly with salt, and allow to cook about 1 minute more. Place a slice of cheese on top of each patty. Cover and cook until cheese starts to melt and burgers are about medium, 1–2 minutes more. Transfer to sheet pan with onions and set aside.

Toast buttered buns on griddle until golden brown, 1–2 minutes (watch them carefully!) Transfer to sheet pan with onions and patties.

Spread secret sauce on the bottom and top bun. Top bottom bun with pickles and patty, then grilled red onion and lettuce. Add top bun to close and serve.