I've made this recipe dozens of times—for big crowds and small get togethers—and it's always a hit. I love it because it's easy to prep ahead of time, so when it's time to eat, all I need is about 5 minutes and a hot griddle.
Thinly slice the onion using a mandoline slicer or sharp knife. Transfer to a sealable container.
Boil the vinegar and sugar in a small saucepan until the sugar dissolves. Pour over the sliced onions. Refrigerate until ready to serve.
Cut the flank steak in half lengthwise, then thinly slice it against the grain. Whisk the brown sugar, soy sauce, garlic, and crushed red pepper together. Place the sliced steak in a resealable zip-top bag and pour the marinade over it. Refrigerate for 30 minutes, or until you're ready to cook.
Whisk the mayonnaise, gochujang or sriracha, and lime juice together and refrigerate until ready to serve.
Place a griddle insert into the grill and prepare the grill for Direct Medium-High Heat cooking (450°–500°F) for 15 minutes.
Preheat the griddle on medium-high heat for 10 minutes.
Place a griddle insert into the grill and set to Manual Mode level 8. Preheat for 15 minutes.
Spread a thin layer of vegetable oil on the griddle. Add the marinated beef in a single layer. Cook, turning occasionally, until browned and cooked through, 5–7 minutes.
Transfer the beef to a platter and let rest for a few minutes.
To serve, spoon a small amount of the mayonnaise onto each lettuce leaf, top with beef and a few pickled onions.
Recipe Tips
If you struggle to slice the beef thinly, try placing it in the freezer for 30 minutes. And feel free to dial the heat up or down by using more or less gochujang or sriracha, depending on how spicy you like things.
