Prep the meat: Divide 12 ounces Impossible Beef burger meat into 4 equal-sized portions. Loosely form into thick, puck-shaped patties (at least ¾-inch thick).Generously sprinkle salt and black pepper on both sides of each patty.Note: Avoid overworking the meat or packing the meat too densely, as it will make it more difficult to smash the burger into a thin, craggy patty.
Slice the onion: Peel and very thinly slice ½ small white or yellow onion into half moons. You should have about 7 to 8 pieces of onion per patty.Top each patty with the onion slices.
Cook the smash burgers
Set aside two large, identical plates. Place a large cast-iron skillet over medium-high heat with 1 tablespoon of oil. When the oil is shimmering and beginning to smoke, carefully transfer two of the patties to the skillet, onion side up.Immediately smash down each patty with a cast-iron grill press or a metal spatula until very thin, about the width of the burger buns. Season the top with a pinch more salt.How to use a metal spatula: With the metal spatula in your left hand, press down on the patty with the flat part of the spatula. With your right hand, use the handle of the wooden spoon to firmly put pressure on the metal spatula.
Cook, undisturbed, for 2 - 3 minutes, or until the bottom of the patty is evenly browned. Use the metal spatula to scrape the bottom of the patty off the pan, and flip. Season the top (now the browned side) with a pinch more salt.Place 1 slice of American cheese on each patty, and cook for an additional 1 - 2 minutes, until the patty is browned on both sides, the cheese is melted, and the onions are nicely browned. Reduce the heat to low, then remove the patties from the pan and transfer to a plate. Cover with another other plate to keep warm.Note: The caramelization of the patty will cause it to stick somewhat to the pan; use the metal spatula to firmly scrape and flip the patty!
Add the remaining tablespoon of oil to the pan and increase the heat back to medium-high. Once the oil begins to shimmer, add the remaining two patties. Repeat the steps of smashing and cooking them until browned and crispy. Transfer to the plate, then get ready to assemble your burgers.
Toast the buns
While the burgers cook, toast the buns. Set another large cast-iron pan, skillet, or flat-top griddle over medium heat and melt 1 tablespoon unsalted butter.Once melted, coat 4 hamburger buns in the butter and arrange evenly in the pan, cut-side down. Toast the buns for anywhere from 1 - 3 minutes, or until they're nice and golden-brown but still soft. You could also cook the buns in the burger pan once they are done.
Assemble the burgers
Remove the buns from the pan and assemble the burgers. Spread the smashburger sauce (or sauce of your choice) on the top burger bun. Top with dill pickles. Top the bottom burger bun with the lettuce and the burger patty with the onions.Stack the burger together and serve warm.
Video
Notes
If you're having trouble smashing the patty in the pan, you can try doing so before adding it to the skillet. Per commenter Ann's advice, roll the meat into a ball, then slip it between two sheets of parchment (or a ziploc bag cut open). Smash the patty with the spatula, then remove the sheets, and transfer to the pan.Serve hot! Get all of your ingredients prepped in advance, and make sure to toast the buns while the meat cooks. This ensures you're ready to eat just as the last patties come out of the skillet.