Prepare your ingredients by dividing ground beef into four equal portions and loosely roll into balls. Mix the secret sauce in a small bowl until smooth and set aside. Toast burger buns lightly.
Heat a cast-iron skillet or griddle over medium-high heat. Place a beef ball on the skillet and immediately smash it down to about 1/4 inch thick. Cook for two to three minutes without moving. Flip, add cheese, and cook for another one to two minutes. Repeat for all patties.
Assemble the smash burger by spreading secret sauce on the bottom bun, adding lettuce, tomato, and the cheese-topped patty. Top with onion slices, pickles, and bacon if desired, then place the top bun on the burger.