In a medium bowl, whisk together the marinade ingredients and then transfer the mixture to a large zip-top bag. Add the steak and onion slices. Turn to coat and seal the bag. Refrigerate for at least 4 hours or overnight. Remove the bag from the refrigerator about 30 minutes before you begin cooking.
Heat a large cast-iron skillet over medium-high heat. Cook the steak slices for 6 to 9 minutes, flipping the pieces to cook them evenly. Transfer the steak to a plate, leaving the juices in the skillet.
Add the onion slices to the skillet, reduce the heat to medium, and cook, stirring constantly, until golden and caramelized, 7 to 9 minutes. Add the steak back to the skillet to heat through.
Make the Mexican Chimichurri:
In a food processor, pulse the jalapeño, garlic, and onion several times, until chopped. Add the cilantro and parsley and pulse again. Add the oil, lime, cumin, and salt. Pulse until combined.
Use immediately or transfer to an airtight container and refrigerate for up to 1 week.
Assemble the tacos:
Assemble each taco by layering the steak and grilled onion on a tortilla, then topping with Mexican chimichurri and pico de gallo. Serve with pickled jalapeños and onions on the side, if desired.
Notes
If you prefer grilled steak, don't slice it first. Grill the steak for 5 to 7 minutes per side to reach your desired doneness. Then slice the steak thinly against the grain and assemble the tacos.