In a medium bowl, squeeze 1 lime, add 2 tsp finely chopped garlic, 1½ Tbsp taco seasoning, 1 Tbsp oil, and a pinch each of salt and pepper. Stir to combine.

Add the shaved steak to the marinade and set aside

Cut the onion in half and then thinly slice.

In a small bowl, whisk 1 Tbsp oil, 1 Tbsp vinegar, a pinch of sugar, salt and pepper. Add ½ cup of the sliced onions and toss to coat. Set pickled onions aside, stirring occasionally.

Heat a large nonstick skillet over medium high. Add 1 tortilla at a time until warmed and lightly golden, about 30 seconds per side. Transfer to a plate; cover or wrap in foil as you go to keep warm. (You can also toast tortillas individually over a gas flame until lightly charred in spots, 5–10 seconds per side.)

Heat 1 Tbsp oil in same skillet over high. Add reserved sliced onions and cook until softened and lightly browned, 2–3 minutes. Using a slotted spoon, add shaved steak and cook, breaking up into large 2-inch pieces, until browned on one side, 2–3 minutes.

Add the marinade from bowl and cook, stirring once or twice, until steak is cooked through, 1-2 minutes. Fill tortillas with shaved steak, then top with pickled onions and whatever other toppings you like.