Slow cooker beef stroganoff

Name: Slow cooker beef stroganoff
Prep time: 20 min
Cook time: 8 h 30 min
Total time: 8 h 50 min
Cuisine: European, Eastern European
Ingredients: 750g stewing steak such as chuck, cut into strips; 1 tbsp vegetable or rapeseed oil plus a little more if needed; 2 onions halved and sliced; 2 garlic cloves crushed; 1 beef stock cube or 1 tbsp liquid stock; 1 tbsp Dijon mustard; 50g butter; 200g chestnut mushrooms halved; 2 tsp cornflour; 200g soured cream; small bunch parsley chopped tagliatelle or mashed potato and a pinch of paprika, to serve