Gather your ingredients.
Chop onions and slice potatoes.
Add potatoes to a bowl of cold water with ice for 20-30 minutes. This helps make them crispy.
Place potatoes in air fryer and coat with oil, salt and pepper. Air fry for 20 minutes at 400°F.
Add onions to an oiled pan and cook over low to medium heat until caramelized.
Remove onions from pan and add burger patties. Smash them down in the pan to make smash burgers and add mustard on top.
Cook to desired doneness, adding cheese after flipping the burger patties.
Make the sauce by combining 2 tbsp mustard, ketchup, relish and mayonnaise.
Toast the buns and slice the lettuce and tomatoes.
Spread sauce on each bun, add the burgers, toppings and caramelized onions.
Add sauce and onions to the fries as well if you want to make them Animal style! Enjoy!