Divide ground beef into 4 equal portions (do not patty them) and chill in the fridge for 30 minutes before cooking.
Meanwhile, heat a large skillet (10- to 12-inch diameter) over medium-low heat. In batches, lightly toast the insides of the buns until golden to golden brown, about 2 minutes (brioche toasts fast, so don’t walk away). Lay them toasted side up on the counter or cutting board.
Take the beef portions out of the fridge. Turn up the skillet heat to high, pour in a light slick of oil (about 2 teaspoons) and turn the pan to make sure it gets coated evenly. When hot, place 1 beef portion in the center of the pan and smash it down in the middle with a spatula. Use the spatula to flatten it more, until about ¼-inch thick. Rough edges are good; just try to avoid rips and big cracks. Cook until the bottom sides are well-browned, about 2 minutes. Season with salt and pepper, then turn the burger over and cook the other side until you’ve reached your desired doneness, about 1 to 2 minutes for medium. Transfer to a plate and repeat with the remaining beef portions.
Build your smashburgers with lettuce, tomato, cheese and condiments of your choice. Serve immediately.