First, we'll prepare the diced onions. Heat 1 tbsp of olive oil in a large skillet on Medium-High heat. Then add the diced onions and salt, and stir them. Reduce heat to medium-low and allow them to cook for about 15-20 minutes while stirring occasionally. Once they're lightly browned, turn the heat back up to medium-high, add 1 tbsp of water, and allow them to cook for another 2-3 minutes until the water evaporates or until the onions begin to stick to the skillet. Repeat this 2 more times, and store in a container until you're ready to scoop them onto the beef patties.
For the Beef Patties
First, lay out a large piece of wax paper. Separate the beef into 8 pieces, and roll each piece into a ball. Flatten out the beef balls using your hands, then lay them on the wax paper, forming them into patties. You want them to be thin- about 4 inches in diameter- or a little bigger than the bun. Repeat with all 8 beef balls and season them with salt and pepper. Cut them into squares and stack them on a plate.
Lightly oil a large skillet and heat on Medium-High for a few minutes before adding the patties. Place 2 patties on the skillet with the seasoned side down, and season the top with salt and pepper and 1 tbsp of mustard. Cook these patties until browned- about 2 to 3 minutes. Flip them and cook them for about another minute.
While still on the skillet, add sliced cheese to both patties. Then add 1 heaping tbsp of caramelized onions onto one patty, and stack the other patty on top. Remove the stack from the pan, and repeat with the remaining patties.
For the Special Sauce
Add all of the sauce ingredients into a bowl. Whisk until well combined. Cover, and store in the refrigerator until ready to use.
For the Buns
Heat a skillet on Medium-High heat, add about 1 tsp of butter per bun. Toast the inside of the buns for about 1 minute, until the edges are dark brown, and set aside.
For Assembly
Assemble the burgers in this order: Bottom bun, 1 heaping tbsp of sauce, 4 pickle slices, 1 tomato slice, 2 lettuce leaves, double meat-cheese-onion stack, and finally the top bun.
Notes
The burger patties are very thin when prepping, so keep them on the wax paper until you are ready to flip it onto the pan.
The caramelized onions take the longest to prepare, so you can make them ahead of time and keep them in the refrigerator until you're ready to use them.
The sauce tastes better the longer it sits, so we recommend making it ahead of time as well.