Make the quick-pickled onion at least 30 minutes before dinner. Pack the sliced red onion into a heat-safe bowl or jar. Bring the vinegar, water, sugar, and salt just to a simmer, then pour the mixture over the onion. Press the onion beneath the liquid and let it cool. Refrigerate until needed.

Pat the steak dry. Rub it on both sides with the neutral oil and Carne Asada Seasoning. Let it stand for 20 to 30 minutes while the toppings are prepared; do not leave it at room temperature longer than 1 hour.

Prepare the cabbage or lettuce, avocado, salsa, cilantro, lime wedges, and drained pickled onion. Keep the toppings cold until serving.

Heat a cast-iron skillet or grill pan over high heat until very hot. Cook the steak without moving it for 3 to 5 minutes per side, depending on thickness and preferred doneness. Flank steak is best around medium-rare to medium rather than well-done.

Transfer the steak to a cutting board and rest it for 8 to 10 minutes. Warm the tortillas in the same dry skillet for about 20 seconds per side, then wrap them in a clean towel.

Identify the direction of the steak's grain and slice thinly across it. Cutting against the grain matters more than almost anything else for tender flank or skirt steak.

Fill each warm tortilla with steak. Add cold cabbage or lettuce, salsa, avocado, cilantro, pickled red onion, and a squeeze of lime. Serve immediately.