Preheat oven to 180°C. Heat oil in a medium non-stick frying pan over medium heat. Cook leeks and garlic for 3–4 minutes.
Add chicken and mustard and cook for 5–6 minutes or until chicken browns. Sprinkle cornflour over chicken mix and cook for 1 minute. Gradually add milk, stirring until mixture thickens. Bring to the boil. Add spring onions and dill and season with salt and pepper.
Place mixture into a lightly greased medium 6 cup capacity baking dish. Lightly spray filo sheets with oil. Fold in half and place over chicken mixture. Tuck edges and top with cheese.
Bake for 10–15 minutes or until pastry is golden and crisp.