Preheat the oven to 190°C fan.
In a deep pan, fry the diced pancetta for 3–4 minutes until starting to turn golden and release its fat.
Add the sliced leeks and celery, season with salt and plenty of black pepper, then pop a lid on and gently cook for 8–10 minutes until lovely and soft.
Stir in the garlic for the final minute.
Sprinkle over 1 big heaped tbsp flour, mix well, then slowly pour in the milk, stirring as you go.
Crumble in the chicken stock cube, add the Dijon mustard and bring to a gentle simmer until thickened and creamy.
Fold through the peas, shredded chicken and tarragon, allowing everything to bubble together for a few minutes.
Taste for seasoning, then leave to cool slightly.
Grab your pie dish and tip in the filling.
Unroll the pastry and cut to fit your dish.
Lay over the filling, crimp the edges and cut a small air vent in the middle.
Brush generously with beaten egg and bake for 25–30 minutes until beautifully puffed and golden, with the filling piping hot.
Serve with plenty of greens and enjoy.
