Preparation

1. Heat a HexClad Double Burner Griddle over medium heat. Brush cut side of buns with softened butter and place on griddle, cut side down. Cook until lightly toasted, 1 to 2 minutes. Remove to a plate. 
2. Divide beef into four 4-ounce balls. Raise griddle heat to medium-high. Place each ball onto the hot griddle and use a large flat spatula to smash the meat down as thin as possible (you may need to use a second wooden spoon or spatula to press down on the beef). Season the patty generously with salt and pepper and top each patty with ½ cup of onions (it’s not only ok if the onions spill over the side of the patties–it's encouraged!). Season onions with salt and pepper. 
3. Cook, undisturbed, until edges of beef are crispy and underside is nicely golden, about 2 minutes. Flip the patties, onions and all, and top with cheese. Cook until onions are golden and cheese is melted, 1 to 2 minutes more.
4. Spread special sauce over both sides of the buns. Top bottom bun with iceberg lettuce. Slide burger patties directly onto the buns and top with tomato and avocado slices. Serve immediately with more special sauce.