Grease the bottom of a slow cooker with butter or oil spray.
next, caramelise the meat (this is optional but taste nicer!).
In a large sauté pan, over high heat, drizzle some oil and sear the meat in small batches for 2 minutes until one side is coloured. transfer each batch in the slow cooker.
In the same pan, over high heat, colour the onions and again transfer in the slow cooker with the meat.
Add the garlic, porcini mushroom powder if using, the mustard, Worcestershire sauce, paprika and 200ml of beef stock. Add the mushrooms at this stage or halfway through the cooking time for firmer mushrooms results.)
Cover and cook on high for 2 to 4 hours or on low for 6 to 8 hours, until the beef is cooked through and tender.
Once nearly ready, in a bowl, combine the corn flour with the cold 100ml of beef stock and pour the mixture over the simmering beef mixture. Cover and cook on high for 15-20 minutes to allow the cornflour to thicken, and then stir in the natural yogurt.
Cover again and cook for a further 5-10 minutes on high.
Check the seasoning and add a few gherkins if desired.
While the stroganoff finishes cooking, cook the noodles or rice according to package instructions.
Stir into the slow cooker or pile separately into bowls and ladle the stroganoff over the top. Serve immediately, topped with gherkins or parsley as desired.