For the chicken & leek filling
In a pan, melt butter over medium heat. Add leeks and cook gently for 8 – 10 minutes until soft. Stir in garlic and sprinkle in flour; cook for 1 – 2 minutes.
Slowly add chicken stock, milk and then cream, stirring to make a smooth sauce. Stir in mustard and tarragon, and season with salt & pepper.
Add chicken, then simmer gently until thickened. Cool slightly.
Assemble & Bake
Preheat oven to 200°C (180°C fan)/ Gas mark 6
Roll out your pastry (shop bought or homemade) to fit your pie dish. Spoon in the creamy chicken and leek filling.
Brush the pastry with melted butter, arrange on top of the pie, and bake for 30 – 35 minutes until golden, flaky, and bubbling.