Mix the mayonnaise, ketchup, relish, onion, vinegar and salt in a small bowl, then cover and refrigerate prior to serving.
Preheat a large, non-stick pan over medium heat for 2-3 minutes. Add 1 tablespoon of oil to the pan, tilting and rotating the pan to distribute the oil evenly.
Add the buns in batches, toasting for 1-2 minutes on one side only, until they develop golden brown edges. Transfer to a cutting board and set aside.
In the same hot pan, add the remaining oil and then the burgers. Sear for 4-5 minutes on each side, until lightly browned, then turn off the heat and keep warm in the pan.
<p style="font-family:trade-gothic-next-condensed;color: #154732">To assemble the Big MacFry Burger, start from the bottom up, on the cutting board. On the first bun, add a tablespoon of the sauce, some red onion slices and shredded lettuce. Next place a burger patty, followed by a slice of cheese and some pickles, then a bun. </p>
<p style="font-family:trade-gothic-next-condensed;color: #154732">To complete this double decker burger, on top of the second bun, repeat by adding the sauce, onion, lettuce, burger, cheese and pickles, followed by the third and final bun. </p>