Slice beef into 1cm strips.
Place a deep,heavy dish on a medium heat,melt butter and fry onion until soft. Add paprika and cook for a couple of minutes,being careful not to let the onion brown.
Add mushrooms and cook for a further 3-5 minutes,remove contents and set aside. If pan is dry,add a dash of cognac or stock to finish and deglaze.
Season beef with salt and pepper,return pan to heat,add half the olive oil and cook beef in two or three batches - you want to seal and caramelise the meat,not stew it.
Remove beef and set aside.
Return mushroom and onion mix to pan,pour in cream,bring to the boil and simmer for a minute until it starts to reduce. Return steak to pan with lemon juice and some parsley.
Serve with tagliatelle.