We're certainly not the first to try to replicate this recipe, but we think we've come pretty close — and closer than others — to nailing it.
We'd like to give a special thanks to Robin Joss of BigRedKitchen for helping us develop this recipe.
In a medium sized bowl, mixed together the beef and chuck, carefully so that you do not overmix the meat. On a clean surface or a cutting board, lay out 1 sheet of parchment paper. Place the beef in the center and top with another sheet of parchment paper. Roll out the beef until very thin, about ¼-inch thick, into a rough rectangle that is about 7-by-10-inches. Trim the edges of the square and use the scraps to pat into a neat rectangle that is 6-by-9-inches.
Season the hamburger square generously with salt and pepper. Using a spatula, cut six 3-by-3-inch patties out of the square. Using a straw, poke 5 holes in each patty, one near each corner and one in the center.
In a 12-inch skillet, melt the butter. Add the diced onion, season lightly with salt, and sauté in the butter for 1-2 minutes until it begins to sweat. Spread the onions evenly along the bottom of the pan so that the bottom of the pan is covered. Turn the heat to medium-low, and top the bed of onions with the patties, 4 at a time, seasoning side down. Allow the onions to "steam" the patties. Cover and cook the patties until they are no longer pink and cooked through, about 4 minutes.
Meanwhile, dampen a kitchen towel and wrap the 6 buns in it. Place in the microwave for 10-20 seconds to steam the buns.
To assemble the burgers, take each patty with no more than 1 tablespoon of the cooked onions and place on the bottom ½ of the bun, onion side up. Top with 1 pickle slice and the top ½ of the bun. Serve with ketchup, if desired.