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Freeze the beef for 45 mins before you begin, slice as thinly as you can, then season.
In a large, non-stick frying pan, melt half the butter with half the oil. Increase the heat, then quickly sear the beef in batches until browned on both sides. Remove the meat and set aside. Repeat with the mushrooms, then set aside with the beef. Boil the pasta.
Add the remaining butter and oil to the pan and soften the shallots for a few mins. Stir in the flour for 1 min, then gradually stir in the stock. Bubble for 5 mins until thickened, then stir in the mustard, purée, crème fraîche and seasoning. Bubble for 1 min more, then return the beef and mushrooms to the pan.
Drain the pasta, toss with half the parsley, season, then serve with the creamy stroganoff, sprinkled with the remaining parsley.