In a small bowl whisk together the mayonnaise, ketchup, pickle relish, red onion, white vinegar and sugar.
Cover and refrigerate until ready to use. The spread can be made up to 1 week in advance.
For assembly and serving:
Press 1.2 oz (32g) of the ground beef onto each flour tortilla. Make sure the meat is spread thinly and evenly across the tortilla right to the edges. Spread about ½ teaspoon (3g) of yellow mustard evenly over each pressed patty. Season with salt and pepper.
Heat a large cast iron pan over high heat. Once the pan is hot, work in batches to cook the tacos depending on the size of the skillet. Add the tortilla patties, patty-side down onto the hot pan and use a metal spatula or heavy item to smash the tortilla flat onto the pan.
Cook for 1 minute then use the metal spatula to scrape the patty and flip the tortilla to warm it for just a few seconds.
Remove to a platter and immediately top with a slice of cheese. Repeat with the remaining meat and tortillas.
Return the pan to heat, add the diced onion to the pan and sauté in the rendered beef fat until the onions are soft and start to brown, stirring occasionally, about 8 minutes.
Top the burgers with a spoonful of the spread, grilled onions, pickles, a slice of tomato and the shredded lettuce. Serve hot.