1. Put the chicken in a large pot, add the vegetables & herbs & salt.
  2. Poach very gently for around 1½ hrs, skimming off any scum that comes to the surface. Once cooked, lift out the bird & allow to cool.
  3. Strain the stock through a fine sieve & discard all solids.
  4. Leave to settle & lift off any surface fat with several sheets of absorbent kitchen paper.  Set aside until you are ready to make the sauce.
  5. Remove all the meat from the chicken carcass (discarding skin & also removing all sinews from the drumsticks) & cut into large, bite-sized pieces.
  6. To make the sauce, melt the butter in a pan, add the flour & cook for 3 mins without browning. Gradually add the hot poaching stock, white wine & cream & stir until thickened.
  7. Stir in the cheese, mustard & tarragon, correct the seasoning & simmer all together for about 20 mins.
  8. Preheat the oven to 230C/450F/Gas 8. 
  9. Put the chicken in a buttered gratin dish, pour over the sauce & sprinkle with the breadcrumbs & Parmesan cheese.
  10. Bake in the preheated oven for 20-25 mins until the dish is golden brown & bubbling well around the edges.
  11. Eat with buttered new potatoes & a crisp green salad.