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Turn on the slow cooker. Season the beef. Heat the oil in a wide frying pan and add the beef in batches so as not to overcrowd the pan. Cook over a high heat until deep brown, then transfer to the slow cooker. Add the onion and garlic to the pan and cook for a few mins until soft, adding more oil, if needed. Add a splash of water and scrape any bits from the bottom of the pan. Tip everything into the slow cooker.
Add the stock to the beef, then the mustard and season. Add enough water to just cover the beef, put the lid on and cook on Low for 6-8 hrs or High for 5-6 hrs.
About 30 mins before serving, melt the butter in a frying pan and cook the mushrooms until soft and caramelised. Tip into the slow cooker. Mix the cornflour with a little liquid from the stroganoff in the frying pan until smooth, then add about 100ml more and bring to a simmer. Once thickened, pour into the slow cooker with the sour cream. Turn to High (if not already) and cook for another 20 mins. Stir through the parsley. Serve with tagliatelle or mash, sprinkled with paprika.