Heat the olive oil in a large heavy bottomed saucepan

Add the leek and chicken

Stir until the chicken is white and the leeks are soft

Add the bacon lardons

Stir and cook for a further 5 minutes

Stir in the tablespoon of flour and coat the meat and vegetables

Add the milk stirring all the time until the sauce thickens

Add the tarragon

Add a little more milk if more liquid is required and stir in well

Preheat the oveb to 190 degrees c

Allow to simmer for 5-10 minutes

Spoon the mixture into a pie dish

Cover with the puff pastry sheet, cutting off any excess with a knife

Bake in the oven for about 45 minutes or until the pastry is risen and golden