Halve each chicken breast width ways and then slice length ways. Quickly seal these in the hot olive oil in a large frying pan and add the onions and tarragon cooking for another 2 - 3 minutes until the onions begin to soften. Add the stock and white wine and bring to the boil. Turn down the heat to a high simmer and reduce until about half the volume has evaporated which may take up to half an hour. Add the cream thoroughly warm through and pour over individual beds of tagliatelle (preferably fresh) in 4 bowls.
Serve with a fresh green salad (water cress and fresh baby spinach) with a garnish of tomatoes and a piquante dressing - something like this mustard one
Mix the first three ingredients together with a fork whisking well. Then add the olive oil a very little at a time whisking it in thoroughly before you add the next splash. This takes time but is more than worth it. You can use an electric gadget but in my experience it's not as good, and if I say that as someone who is an absolute gadget freak and technophiliac it might just be true.