Add the oil to a frying pan and heat it over medium-high heat until hot.
Add the sliced onion to the pan and cook slowly, stirring occasionally, until the onions are deeply golden.
Once browned, remove the onions from the pan and set them aside for later.
Place a small portion of beef directly into the hot pan, then immediately press a mini tortilla on top of the beef and firmly smash it down so the beef spreads evenly underneath.
Let it cook undisturbed for 3–4 minutes, or until a deep golden char forms on the bottom of the beef.
Carefully flip the tortilla and beef over so the tortilla is now underneath, and immediately reduce the heat to low to avoid burning.
Generously season the beef with salt and freshly ground black pepper while it’s still hot.
Quickly add a cheese slice on top of the beef, followed by your desired toppings, including the onions, lettuce, pickles, tomato, and a drizzle of burger sauce.
Let it sit just long enough for the cheese to melt slightly.
