Heat a large, heavy-based saucepan over high heat. Season meat with salt and freshly ground black pepper. Add the oil and a handful of beef strips to the pan, and brown all over, remove to a side plate. Repeat in batches. Add more oil and add the onion and garlic and cook for 2-3 minutes until onion is soft.
Add the mushrooms, thyme and salt and pepper, and stir to combine and cook for 3 mins. Return beef strips to the pan along with the tomato paste and mustard and cook for a further 3mins on a medium heat. Remove from heat and fold in sour cream and parsley. Serve with wide ribbon pasta (fettuccine or pappardelle), mash or rice.
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