Slice beef into bite-sized pieces and sear quickly in a hot pan to get some colour all round - remove and set aside
Add 2tbsp unsalted butter to the pan, then cook 250g sliced chestnut mushrooms until water is cooked out and they char/brown, then set aside
Add 1tbsp butter to the pan + 1 finely sliced shallot, 2 cloves minced garlic and touch of thyme - sweat down for 3-4 mins, then add back the mushrooms
Sprinkle over 2tbsp AP flour, cook off ~1 min, then deglaze with white wine - allow alcohol to cook out ~3 mins
Add beef stock or demiglace ~500ml
Leave to simmer and reduce by half, then add 150ml double cream, 1tbsp Dijon, 2tbsp Worcestershire - simmer and reduce till thickened
Finish sauce with 2tbsp sour cream
Cook pasta in salted boiling water, once al dente, strain and add 2tbsp salted butter and meld with 2tbsp pasta water till melted
Add beef back to the sauce, heat through and season to taste
Plate up and top with the sauce and chopped parsley
