In a large bowl, gently mix the burger patty ingredients together with your hands until just combined- we don't want to over-work the meat, as it will become tougher the more we handle it. Divide into 4 equal portions but do not form patties.
In a small bowl, mix all smashing sauce ingredients together and set aside.
Heat a cast-iron griddle or large heavy skillet over medium-high until very hot, about 2 minutes. Lightly brush with vegetable oil. Place portions of meat onto the griddle (you might have to do this is batches or use two pans) and smash flat with a spatula to form 4"-diameter patties. Rough edges are a good thing here- they will provide extra caramelized bits! Cook without flipping until outer edges are brow- roughly 2 minutes. Flip patties and place a slice of cheese on top of each patty, if desired. Cook until cheese melts and burgers are medium-rare, about 1 more minute.
Lightly toast buns and give both sides a healthy slather of the extra-special sauce. Top with a burger patty, lettuce, onion and tomato. Best enjoyed outside with a cold drink and a biiiig napkin!
Notes
* Can be substituted with ½ tsp, ½ tsp pepper and ¼ tsp garlic powder ** Yep, that's right. I use two different type of pickles in this recipe! Bread and butter pickes are so sweet they work perfectly in the smashing sauce but I find them a little too much to top a burger with. As a topping, I prefer a crisp dill pickle- better crunch and more tang to cut the richness of the beef.
Recipe by Port and Fin at https://portandfin.com/old-school-beef-burgers-with-extra-special-sauce/