Add the buns cut-side-up to a baking tray and place under the grill until crisp & golden. Prep all the other ingredients (make burger sauce, slice veg etc).
Cut the beef into 4 pieces and gently roll each one into a ball (doesn't have to be perfect).
Turn on the extractor fan and consider opening a window, then place a cast-iron pan over high heat. Once smoking hot, add 1-2 tsp oil then add in 1-2 portions of beef. Quickly add a square of baking paper over each one and use a spatula with your hand to very firmly press down on the balls until they flatten. You want them relatively thin (5-7mm), but not so thin they break apart.
Remove the baking paper and give them both a generous seasoning of salt and black pepper. Cook for 90 seconds, then flip them over. Give them a more modest seasoning on the cooked side then top both with cheese. Cook for another 90 seconds then remove and repeat. Don't push down on the burgers after you've smashed them otherwise the juices will pour out. You also won't need extra oil for the second batch.
For the regular smash burger stack in this order: bottom bun, burger sauce, lettuce, tomato, onion, 2 patties, gherkins then top bun with more burger sauce.
For the Dad burger stack in this order: bottom bun, mayo, lettuce, tomato, patty, onion chutney, bacon, middle bun, mayo, lettuce, jalapeños, patty, onion rings, BBQ sauce, top bun.