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For the chimichurri butter, tip everything into a medium bowl, season, then beat with a wooden spoon or spatula to combine. Scrape onto a sheet of baking parchment, then roll into a sausage, enclosing the butter in the parchment, and twist the ends to seal. Chill for 1 hr until firm. Will keep chilled for three days or frozen for a month.
Heat the oven to 110C/90C fan/gas ¼, or its lowest setting. Put the beef on a baking tray and cook for 2 hrs 30 mins-3 hrs, or until the internal temperature reads 55C when tested using an instant-read thermometer. If you don’t have one, press the beef with your finger – it should feel soft for rare, lightly springy for medium-rare and firm for well done. (See tip, below.)
Remove the beef from the oven. Heat the oil and butter in a large frying pan over a high heat until the butter is foaming. Season the beef generously with salt and cook in the pan for about 4 mins, or until deeply golden. Turn over and repeat on the second side. Melt a few thick slices of the chimichurri butter over the steak as it cooks. Serve immediately in slices alongside fries and a rocket salad (see below), with any extra chimichurri butter on the side.