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If your barbecue hasn’t already been heated, light and heat until the ashes turn grey, or heat a griddle pan. Rub the beef with 1 tbsp olive oil and the garlic, season well with salt, then coat each side with the crushed peppercorns. Put on the barbecue or in an almost smoking griddle pan. Brush the top with the rest of the oil using the thyme, then place the thyme on top. Once the underside is charred with a nice crust (about 5 mins), remove the thyme and set aside, turn the fillet over and cook for about 5 mins more.
Test the temperature of your fillet using a meat thermometer to make sure you don’t overcook it. For medium rare, you want it between 52-55C. Once at the desired temperature, remove the meat to a board, place the thyme and garlic on top and cover in foil. Leave to rest for about 5 mins.
Meanwhile, make the sauce by combining the soured cream and mustard in a bowl. Season and set aside. Transfer the beef to a clean board, slice into 2-3cm thick pieces and serve with the sauce.