Start by melting butter in a pan and gently cooking the leeks until soft. I make sure not to brown them, as this keeps the flavor delicate. Once softened, stir in the flour and cook briefly to remove the raw taste.
Gradually add chicken stock while stirring to avoid lumps. I like to keep the heat medium so the sauce thickens evenly. Then pour in the cream and mustard, letting everything simmer into a rich, smooth mixture.
Mix in the cooked chicken and ensure it’s fully coated with the sauce. I always taste at this stage and adjust salt and pepper as needed. Let the filling cool slightly before transferring it to a baking dish.
Place the filling into your dish and cover it with puff pastry. I gently press the edges to seal and trim any excess. Brush the top with beaten egg to give it that beautiful golden finish.
Bake in a preheated oven at 200°C (180°C fan) until the pastry is puffed and golden. I usually check after 25 minutes, but it may need a bit longer. Let it rest for a few minutes before serving.