Thinly slice the 1 1/2 lb flank steak across the grain and place the slices in a medium bowl.

In a separate bowl, stir together the marinade: 4 Tbsp sesame oil, 4 garlic cloves (minced), 1 Tbsp fresh ginger (grated, optional), 1/4 cup soy sauce, 1/4 cup packed light brown sugar (press it into the cup), 1 tsp ground black pepper, and 1/4 cup hot water. Stir until the sugar dissolves.

Pour the marinade over the sliced beef and stir to coat. Cover and refrigerate for at least 4 hours or overnight for best results.

About 30 minutes before cooking, remove the marinated beef from the refrigerator to come to room temperature.

While the beef comes to temp, make the taco sauce: stir together 1/3 cup sour cream, 1/4 cup mayonnaise, 1 tsp garlic powder, 1 tsp Sriracha hot sauce, and 2 Tbsp fresh lime juice. Transfer to a zip-top bag and snip off a corner for easy drizzling, or keep in a small bowl. Refrigerate until ready to use.

Prepare toppings: thinly slice or shred the 1 small purple cabbage; pick the leaves from 1/3 bunch cilantro; slice 1 avocado. Set aside lime wedges if using.

Toast 12 to 14 small, 6-inch flour tortillas (or use corn tortillas for gluten free) on a hot dry skillet or directly on the gas-stove grates, turning frequently until warmed and lightly charred. Keep warm wrapped in a clean towel.

Heat a large, heavy nonstick (or cast-iron) pan over high heat until very hot — a piece of meat should sizzle strongly on contact. Do not add extra oil.

Remove beef from the marinade, letting excess drip back into the bowl. Add the beef to the hot pan in a single layer (work in batches if necessary so the pan isn’t crowded). Sauté without stirring for 2–3 minutes until the bottom is lightly browned.

Stir the meat and continue cooking 3–5 more minutes, stirring occasionally, until the beef is cooked through, most liquid has evaporated, and the sauce has caramelized on the meat. Transfer cooked beef to a plate and repeat with remaining beef if you cooked in batches.

To assemble tacos: place toasted tortillas on a plate, top with the cooked beef, a generous amount of shredded purple cabbage, cilantro leaves, and sliced avocado. Drizzle with the prepared taco sauce and add a squeeze of lime if desired. Serve immediately.