To make the pastry, put the flour, sage, a pinch of salt and grating of nutmeg in a bowl. Rub in the butter using your fingers until sandy. Beat the egg yolk with 1-2 tbsp water. Add most to the pastry, reserving 1 tsp. Mix, then bring together into a dough with your hands. Flatten into a disc. Wrap. Chill for 30 mins.
For the filling, lay the chicken on a board, and with the palm of your hand pressing down on top, cut through the middle lengthways from the side without going all the way through. Open out like a book and cut into six strips. Squeeze the sausagemeat from the skin.
Cut a third of the pastry off and set aside. Roll out the rest of the pastry on a lightly floured surface to the thickness of a 10p coin. Line a 10cm pie dish (about 3cm deep), leaving any excess overhanging. Scatter the breadcrumbs in the base, then top with half the chicken. Season. Spoon in half the cranberry sauce, the sausagemeat, the remaining cranberry sauce, and the rest of the chicken. Season.
Roll the reserved pastry out so it’s large enough to cover the dish. Brush a little of the reserved egg around the edge of the pie, then top with the pastry lid, pressing the edge to seal. Trim away any excess. Brush with the rest of the egg and cut a small steam hole in the centre. Will keep chilled for up to 24 hrs, or frozen for two months.
Cook the potatoes in a pan of boiling water for 5-8 mins until fluffy but still holding their shape. Drain and leave to steam-dry. Heat the oven to 200C/180C fan/gas 6. Toss the potatoes in half the oil on a large baking tray. Season. Bake with the pie for 30 mins.
Push the potatoes to one end of the tray, then add the rest of the veg to the other end. Toss with the rest of the oil and season. Bake for 20 mins more, covering the pie with foil if it darkens too quickly. Remove the pie from the oven and roast the veg for 10-15 mins more until tender and the potatoes crisp. Serve with the gravy and extra cranberry sauce.