In a bowl or resealable bag, combine oil, lime juice, minced garlic, salt, pepper, paprika, cumin, and chopped cilantro.
Add the steak to the marinade, ensuring it’s fully coated in the marinade. Cover and marinate for at least 30 minutes, or up to 24 hours. (If you’re only marinating for 30 minutes, you can leave it covered at room temperature instead.)
In a bowl, mix the mayo, sour cream, oil, red wine vinegar, cilantro, parsley, minced garlic, salt, pepper, red pepper flakes, and cumin until well combined. (This creamy chimichurri sauce can be made 1-2 days in advance.)
When ready to cook, remove the steak from the fridge and let it sit at room temperature for about 30 minutes to ensure even cooking.
Heat a large skillet over medium high heat until hot. Add the marinated steak and cook for 3 minutes per side, or until it reaches your desired doneness.
Transfer the steak to a cutting board and let it rest for 5 minutes before cutting it. (This is important to lock in the juice.)
Slice thinly against the grain or cut into small cubes if preferred.
Warm the tortillas in a dry skillet until soft and slightly charred. Prepare your other desired taco toppings.
To assemble, spread a spoonful of chimichurri sauce on each tortilla, then top with the sliced steak, marinated onions, crumbled feta cheese, and fresh cilantro.
Serve immediately with your favorite sides and enjoy!