Heat the oven to its highest setting. For the fries, pour the sunflower oil into a baking tray and put in the oven for 15 mins to get hot. Reduce the temperature to 220C/200C fan/gas 8. Tip the fries into the oil, turn to coat, and cook for 20 mins. Add the Cajun spice mix and return to the oven for 15 mins until golden and crisp.
Meanwhile, make the mayo by mixing all the ingredients together in a bowl. Set aside.
For the chimichurri-style sauce, blitz all the ingredients together until you have a smooth sauce, then season to taste. Set aside.
Heat a griddle pan over a high heat. Rub the steaks with salt only (pepper will catch and burn), put the steaks in the pan and cook for 3 mins on each side (for medium). Remove from the pan, cover with foil and leave to rest for 10 mins.
Halve the ciabatta rolls, toast in the same griddle pan that you cooked the steak in, cut-side down, and spread one half of each with the chipotle mayo. Slice the steak into strips, divide between the ciabattas, drizzle over the chimichurri and add a little rocket. Serve with the fries and any leftover mayo.