Slow cooker beef stroganoff

Cook a version of beef stroganoff that's lighter on the wallet yet full of flavour thanks to slow cooking. Serve with pasta, rice or mashed potato

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750g stewing steak, such as chuck, cut into strips 1tbsp vegetable or rapeseed oil, plus a little more if needed 2 onions, halved and sliced 2 garlic cloves, crushed 1 beef stock cube or 1tbsp liquid stock 1tbsp Dijon mustard 50g butter 200g chestnut mushrooms, halved 2tsp cornflour 200g soured cream Small bunch parsley Chopped tagliatelle or mashed potato and a pinch of paprika, to serve