Preheat oven to 200°c. Line 2 baking trays with baking paper. Heat oil in a frying pan over medium heat. Cook onion, carrot and celery for 8 minutes or until softened. Add spinach leaves and cook for 2 minutes or until wilted. Transfer to a bowl and cool. Add leftover turkey, leftover stuffing, mustard and cheese to vegetable mixture and gently combine. Season.
Cut 2 x 14cm circles from each sheet of pastry. Spoon 1/3 cup of turkey mixture in the centre of each pastry round and spread, leaving a 2cm border. Brush egg around half the circle border. Bring pastry edges together to form a semi-circle. Pinch edges to seal and form a frill. Brush with egg. Bake for 25 minutes or until golden and puffed. Serve with chutney.