If you're making fry sauce, mix the ingredients together in a small bowl.
Cut up your optional toppings. If you'd like fried onions you can put them in a pan on low heat with some butter for about 20 min.
Wad up a paper towel and rub in your cast-iron skillet to coat with vegetable oil.
Add pan to stove on med-high heat until hot (just beginning to smoke). You may want to turn on a vent fan.
Divide burger meat into 1/4 lb pucks (a couple inches wide & tall). Salt & pepper one side.
Add meat to pan, spiced side down and immediately smash with spatula (or two) until it's about the size of the bun.
Top the 2nd side with salt and pepper. Cook for about 1-1.5 mins and then flip the burger (don't smash again). Finish cooking for about 30 secs to 1 min for medium.
Add to your bun and top how you'd like. Enjoy!