Preheat the oven to 220°C, fan 200°C, gas 6. Heat the oil in a large pan, add the leeks and carrots and cook for 3-4 minutes until lightly golden. Add the sage and 3 tbsp water, cover and cook gently for 8 minutes, until the carrots are soft. Add the chicken and heat through, then stir in the mustard and crème fraîche and warm for 2 minutes.
Divide the mixture equally among 4 ramekins or ovenproof dishes (approximately 300ml each). Roughly scrunch a piece of filo pastry and lay it on top of the filling to cover. Brush with sunflower oil and bake for 12-15 minutes until crisp and golden.