White Castle was America’s first fast food hamburger chain, and it’s often credited with launching the country’s burger obsession, with its small square steam-grilled sliders. They’re sold in packs of four, sacks of 10 and even cases of 30. The company calls its fans “cravers” (Harold and Kumar of the 2004 cult movie weren’t the only ones!), and every year new devotees are inducted into the Cravers Hall of Fame.
Put the onion in a medium bowl and cover with 1/2 cup water. Set aside 30 minutes.
Meanwhile, puree the beef with 2 teaspoons water, the onion powder, 3/4 teaspoon salt and a few grinds of pepper in a food processor until smooth. Draw a 6-by-12-inch rectangle on a piece of parchment paper; flip over the paper so the pencil side is down (you should still be able to see the rectangle). Press the beef into the rectangle; cover with another piece of parchment and press until the beef is very thin and fills the rectangle completely. Uncover and score the beef into eight 3-inch-square patties. Poke 5 holes into each patty with a chopstick (one in the center and one near each corner). Slide the beef on the parchment onto a baking sheet, loosely cover with plastic wrap and freeze until firm, 10 to 15 minutes.
Trim the edges of the rectangle to make it even; cut patties along the scored lines. Heat an extra-large skillet over medium-high heat. Add the onion and liquid; bring to a boil. Stir in the butter and arrange the patties over the onion. Cook until the meat is no longer pink around the edges, 3 to 4 minutes. Put a bun bottom upside down on each patty. To steam the bun tops, rest each across 2 bun bottoms (like a bridge). Continue cooking until the patties are completely cooked through, about 2 more minutes, adding 1 to 2 tablespoons water if the skillet is too dry or if the onion starts browning.
Remove the bun tops from the skillet. Slide a spatula under each patty, picking up the onions and bun bottoms along with the meat; flip over onto a plate. Top each with a pickle and a bun top.