Skillet Smash Burgers with Quick Pickles

Steps

1. Make the quick pickles first: stir the cider vinegar, water, sugar and half the salt in a bowl until dissolved, add the cucumber, and let stand 15 minutes while you prepare the rest.
2. Divide the ground beef into 8 loose balls without packing them. Keep them cold until cooking.
3. Spread the butter on one side of each bread slice and toast butter-side down in a large skillet over medium heat until golden, about 2 minutes. Set aside.
4. Raise the heat to high. Place 4 beef balls in the dry skillet, top each with a few onion slices (using half the onion), and smash flat with a sturdy spatula. Season with half the remaining salt and half the black pepper.
5. Cook 2 to 3 minutes until the edges are deeply browned, flip, and cook 1 to 2 minutes more until the patties reach 160°F (71°C). Lay a cheddar slice over 2 of the patties, then stack each plain patty onto a cheesy one. Repeat with the remaining 4 beef balls, the remaining onion, the remaining salt and the remaining black pepper, adding the last 2 cheddar slices.
6. Drain the pickles. Spread the mustard on the toasted bread, layer on the romaine, a double patty stack and the pickles, then close and serve.