Preheat your oven to the lowest setting and place a baking sheet inside (to keep burgers warm).
To make the special sauce, add all ingredients to a small bowl and mix to combine.
Divide the ground beef into 8 (2 oz) portions and roll each into a ball.
Place a large cast-iron or nonstick skillet over medium-high heat. Once the pan is hot, add about 1 teaspoon oil and carefully rub around the pan to coat.
Place a meat ball in the pan and immediately press it down firmly using a metal spatula to place on top of the patty and a metal measuring cup to smash it down (see video) into a thin, flat patty. Repeat with as many burgers as can fit in your pan (between 2-4). Sprinkle each patty with a generous pinch of salt and pepper and let cook for 2-3 minutes, undisturbed, or until deeply browned. Use a metal spatula to flip each patty (making sure not to lose any of that brown crust) and cook for another 1 minute on the other side.
Top each patty with cheese for the last 30 seconds. Stack one patty on top of the other and transfer to the baking sheet in the oven to keep warm. Repeat with remaining burgers. Toast your buns in the pan.
Add special sauce to bottom bun, top with tomato, lettuce, patties with cheese, and top bun. Serve with German Potato Salad (if desired).