1.Preheat oven to 200°C (180°C fan-forced). Line a banking tray with baking paper.
2.Melt butter in a medium saucepan over moderate heat. Add leek. Cook, stirring, 5 minutes or until soft. Add flour. Cook, stirring, 1 minute. Gradually stir in milk until smooth. Bring to the boil. Reduce heat. Simmer 1-2 minutes, or until sauce thickens. Stir in mustard. Season with salt and pepper. Cool 5 minutes.
3.For each parcel, stack 2 sheets pastry on a flat surface, spraying between sheets with oil. Place one chicken breast along one short end of pastry. Spoon half the sauce over chicken. Fold in short ends, roll up to enclose filling and form a parcel. Place on prepared tray, spray with oil. Sprinkle with seeds. Bake 20 minutes, or until pastry is golden. Serve with salad.
This is a good way to use up left over chicken. Make parcels, brush with melted butter and store in freezer, uncooked, for up to 5 weeks Wash leek thoroughly to remove dirt.