For the marinade mix lime juice, salt, pepper and olive oil together. Marinade for at least 2 hours in the fridge. Let the steak come up to room temperature before cooking.
Heat oil in the fry pan on a high heat. Fry the steak 4 to 5 minutes on each side, depending on the thickness of your steak. Take the steak out of the pan and rest for about 10 minutes.
While the steak is resting make the chimichurri style sauce. Add the red onion to a bowl, add the vinegar and let soak for 2 minutes to soften the onion. Finely chop the herbs, chili and garlic and add together with the olive oil. Finish with salt and pepper to taste.
Serve the steak sliced against the grain, with the chimichurri on the side