Divide the beef into 8 loose balls (about 90 g each) and keep them in the fridge. Lightly salt the tomato slices and lay them on a paper towel. Have the lettuce, pickles and cheese ready by the stove.
Spread the mayo on the cut sides of the buns and toast them cut side down (in the same skillet as the caramelized onions) over medium heat until deeply golden and crisp, 1 to 2 minutes. Set aside.
Grab a clean, dry stainless steel pan! We won't be adding any oil or fat to the pan before smashing down our patties. The 80/20 beef releases plenty of its own fat as soon as it hits the hot pan, and starting dry gives the meat direct contact with the surface for the best possible crust.
Turn the heat to medium high and let the pan get hot. (preheat for 5 minutes). I personally like to do one patty at a time. If I'm making a smash burger, I want a beautiful caramelized bottom, and the best way to get that is to give the patty the entire surface of the pan to itself.
Using a grill/burger press if you have one (if you don't, a wide, sturdy spatula will do) and a square of parchment so it doesn't stick if necessary, smash the patty down as thin as you can get it. It's okay if bits of your burger separate a bit once smashed. Smashing it down in the hot pan creates a caramelized web on the bottom of your patty, and it will all be basically intact once you flip it over.
Season the top with salt and pepper. Cook without moving until the edges are lacy and the bottom is deeply browned, about 2 minutes.
Scrape under the patty to keep the crust, flip, and top with a slice of cheese. Cook until melted, 30 to 45 seconds. Stack the patties in pairs on a plate and tent loosely. Repeat with the remaining beef. If the fond starts looking close to black between patties, lower the heat a little.