2) Put the ancho peppers in a small saucepan with the stock and bring to the boil over a medium heat. Turn off the heat and allow the peppers to steep for 5 minutes. Alternatively, put the peppers and stock in a microwave-safe bowl, cover with cling film, and cook in the microwave for 2 to 3 minutes. Allow to cool slightly.
3) Heat a griddle pan or grilling element to high. Meanwhile, put the warm peppers and onion mixtures in a food processor and puree. Season with salt and pepper, to taste.
4) Season the meat with salt and pepper, to taste, and drizzle with some extra virgin olive oil. Cook over a high heat for 5 to 6 minutes in total, turning once, for rare. Remove the steak to a cutting board and let it rest for 5 minutes.
5) Thinly slice the steak and arrange it on a serving platter. Heat up the ancho-onion sauce, if necessary. Serve the steak with lots of sauce poured over the top.