In a mixing bowl, combine skirt steak with olive oil, lime juice, minced garlic, salt, and pepper. Ensure the steak is well-coated with the marinade. Cover and refrigerate for up to 2 hours.

Slice red onions thinly and place them in a bowl. In a measuring cup, whisk together equal parts vinegar and water with salt until dissolved. Pour this solution over the onions and let soak for at least 30 minutes.

In a food processor, combine fresh parsley, minced garlic, olive oil, lime juice, and salt. Pulse until well-blended but still retains some texture.

Heat a grill or large skillet over medium-high heat. Add the marinated skirt steak and cook for 3-4 minutes on each side, depending on preferred doneness.

Let the steak rest before slicing it against the grain into strips. Warm the corn tortillas, then layer sliced steak onto each tortilla, top with chimichurri, and finish with pickled onions.