Put the chicken in a large pot, add the vegetables , herbs and salt. Poach very gently in stock for about an hour and a half, skimming off any scum that comes to the surface.
Once cooked, lift out the bird and leave to cool. Strain the stock through a fine sieve and discard all solids. Leave to settle and lift off any surface fat using several sheets of absorbent kitchen paper.
Remove all meat from the chicken carcass ( discarding skin and sinew) and cut into large bite sized pieces.
To make the sauce melt the butter in a pan then add the flour and cook for 3 minutes without browning. Gradually add the hot chicken stock, white wine and cream and stir until thickened.
Stir in the Gruyere cheese , mustard and tarragon, check seasoning then simmer all together for about 20 minutes. Preheat the oven to 230 C, Gas 8.
Put the chicken on a buttered gratin dish, pour over the sauce and sprinkle with the breadcrumbs and Parmesan. Bake in the preheated oven for 20-25 minutes untilthe dish is golden brown and bubbling around the edges.