Heat oven to 200C/180C fan/gas 6. Put the potatoes on a baking tray, toss with 1 tbsp of the oil, season and roast for 25 mins.
While the potatoes roast, heat another 1 tbsp oil in a frying pan and cook the onion over a medium heat for about 7-10 mins to soften. Stir in the garlic, chilli flakes and sage and cook for another minute or 2. Remove from the heat and add the chestnuts, cranberry sauce and the sweet potato cubes. Season a little.
Line a 20cm square tin with a rectangle of baking parchment that comes up two sides of the tin (this is to help you remove it later). Put 1 sheet of filo in the tin, brushing the bottom and sides with a little of the remaining oil, then add another piece of filo going in the other direction (like a cross). Brush with more oil on the bottom and sides, and repeat with another 2 sheets of filo.
Spoon the sweet potato mixture into the tin and fold over the filo to cover, brushing with a little more of the oil. Brush the last piece of filo with the remaining oil, then scrunch on top of the parcel. Put in the oven and bake for 30 mins. 5 To serve, use the baking parchment to lift the parcel outof the tin and cut into four.