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First, make the ketchup. Pour 150ml boiled water from the kettle over the dried mushrooms and leave them to soak. When cool enough to handle, drain and give them a squeeze, reserving the water. Tip into a food processor with the portobello mushrooms and pulse until finely chopped.
Heat the oil in a saucepan set over a medium heat, add the shallots and cook for 5 mins until softened and turning golden. Tip the chopped mushrooms into the pan, season and cook for 10 mins, stirring occasionally until all the liquid evaporates. Stir in the sugar, vinegar and Worcestershire sauce and simmer for 5 mins, then pour over the reserved mushroom stock and simmer for 5 mins more until you have a thick mushroom purée. Scrape into a food processor or a blender, and blitz again until smooth. Can be made up to four days ahead and kept chilled.
To cook the beef, heat the oven to 200C/180C fan/gas 6. Heat the oil in an ovenproof frying pan set over a medium heat. Season the beef heavily with salt, then carefully lay the beef fat-side down in the hot oil and fry it to render the fat until golden brown – this will take about 10 mins. Turn it over and fry it on the other other side until browned.
Lift the beef onto a plate, then add the herbs to the pan and sit the beef on top, fat-side up. Transfer the pan to the oven and roast for 30 mins for rare (the core will be 45C on a meat thermometer), 35 mins for medium- rare or 40 mins for medium. Once cooked to your liking, leave the pan on the side to rest for 10 mins.
While the beef cooks, remove the ketchup from the fridge, if you’ve chilled it, so that it comes to room temperature and spoon into a side dish. Once the beef has rested, lift onto a board and carve into thin slices, and serve with the ketchup, watercress and chips, if you like, or your favourite roast dinner sides.