Balsamic Mayo: Place ingredients in a small bowl and mix to combine. Cover and set in the fridge until ready to use.

Make Caramelized Onions: Bring a large cast iron skillet to medium-high heat. Add 1 tablespoon olive oil and move it around, so that the bottom of the pan is coated. Add onions and sprinkle with salt and pepper. Move onions around the pan and bring heat down to medium heat once onions start to cook down. Continue cooking onions 15-30 minutes, until they start to turn golden brown. Once onions are a nice brown color, turn off the heat, and set aside until burgers are ready.

Prep Grill: Preheat the grill to a medium-high heat and make sure the grill grates are clean.

Prep Burgers: Place ground beef, salt and pepper in a mixing bowl and mix with your hands to combine. Divide into 4 equal burger patties. Make a small thumb indent in the middle of each burger. Sprinkle the tops of the burgers with a little more salt and pepper.

Grill Burgers: Place the seasoned burgers directly on the hot grates and cook over direct heat for 3-4 minutes on each side (6-7 minutes total). This will result in a medium-rare center. If you want cheese, place a slice of cheese on each burger for the last 1-2 minutes of cook time. Remove and let rest for 5 minutes.

Assemble Burgers: Place a cooked burger on the bottom bun, add your desired toppings, including some of the caramelized onions and balsamic mayo. Then the top bun.

Serve: Enjoy right away with any of your favorite side dishes!