Start by finely slicing one large onion. If you’re tempted to use a grater, it's best to skip it as it can be more trouble than it's worth. Instead, simply use a sharp knife to get nice thin slices. Preheat a cast iron skillet over medium heat and add 2 tablespoons of butter along with 1-2 tablespoons of olive oil. Once the butter is melted, add the onions. Season them with about 1/2 teaspoon of salt and a pinch of black pepper. Stir the onions and let them cook down until they turn a deep golden brown, which will take about 10-15 minutes. The slow caramelization brings out a rich sweetness that adds a fantastic depth of flavor to your tacos.
While the onions are caramelizing, make your delicious burger sauce. In a small bowl, combine 1/4 cup mayonnaise, 1 tablespoon mustard, 1 tablespoon ketchup, 1/2 teaspoon Worcestershire sauce, and 1 tablespoon pickle relish. Mix it well until everything is combined into a smooth sauce. This creamy, tangy burger sauce will be the secret ingredient that brings all the flavors together.
Take 1 pound (450 grams) of 80-20 ground beef and divide it into roughly 6 equal portions. Place each portion on top of a small tortilla (approximately 15 cm in diameter), pressing it down so that the beef sticks to the tortilla. Remember, the beef will shrink a bit as it cooks, so make sure it’s spread to cover most of the tortilla. Sprinkle each patty with salt and pepper, and then spread about 1 teaspoon of mustard over each patty. This mustard layer will add an extra burst of flavor when it hits the skillet.
Heat your cast iron skillet over medium-high heat. Place each tortilla with the beef side down into the skillet. Press down gently to ensure good contact with the skillet, allowing the beef to cook evenly. Let the patties cook for about 2-3 minutes until the beef starts browning and the edges crisp up slightly. Flip each taco so that the tortilla side is now in contact with the skillet. Allow it to get a bit crispy, which will take about 1 more minute. While the tortilla is cooking, add a slice of cheese on top of each patty.
Once the cheese has melted, remove the tacos from the skillet. To assemble, spread a generous spoonful of the burger sauce over each patty, then add a portion of the caramelized onions. You can top with some freshly chopped lettuce or pickles if you like an extra crunch. Fold the tortillas over to make the classic taco shape, and your Smash Burger Tacos are ready to serve!