Heat oil in a non-stick frying pan over a medium-high heat. Add leek. Cook for 6-8 minutes, or until golden. Transfer to a plate.
Add chicken to pan and cook, stirring often, for 5 minutes, or until browned.
Return leek and add corn to pan with mustard. Stir until well combined.
Sprinkle flour over chicken mixture. Stir until well combined. Remove pan from heat. Slowly add milk, stirring constantly. Return to heat and stir until mixture comes to the boil. Simmer for 1 minute. Remove from heat and cool.
Preheat oven to 200°C Line a baking tray with baking paper.
Place a sheet of filo pastry on a flat surface. Spray with olive oil. Fold pastry short end to short end. Spread 1/3 cup of chicken mixture on edge of pastry. Roll pastry once. Fold in sides and continue to roll until a sausage roll shape is formed.
Place on prepared baking tray. Continue to make rolls with remaining pastry and filling.
Brush rolls with egg and sprinkle with sesame seeds.
Bake for 15-20 minutes, or until pastry is golden and crisp.