I used skinless chicken breast for this but you can use any cut you like.
Cut the chicken into bite size chunks and colour in a sauce pan. add the chopped leeks and sweat for three to four mins.
Add chopped tarragon and a splash of white wine and simmer on a low heat.
I another saucepan melt a knob of butter - take off the heat and add two teaspoons of flour and stir in.
return to the heat and add a dash of milk stirring it in. Keep adding milk a bit at a time and stirring in.
After ten mins or so the sauce should thicken.
Add to the chicken and leeks and simmer until the chicken is cooked through.
Add to a pie dish and allow to cool - top with pastry of choice and brush with milk.
Place in an oven at 180 degrees for 35 to 40 mins or until the pastry has cooked.