Melt half the oil and butter together in a frying pan, then add the onions and cook gently until very soft. Stir in the sage, mace and pepper for 1 minute, then tip into a bowl to cool.
Preheat the oven to 190°C (170°C fan oven) Gas Mark 5. Add the breadcrumbs, one egg and sausage meat to the cooled onions and mix together well.
If you’re using chicken breasts, halve each to make two thin breasts. Heat the remaining oil in the frying pan and fry the turkey/chicken fillets to brown each side.
Roll out the pastry block to line a 22-23cm deep spring-form tin with a little overhang at the top. Press in half of the stuffing to an evenly level base. Dot over the chestnuts, pushing in to stick, then spoon over the cranberry sauce. Add the turkey/chicken fillets to make one solid layer, then finish with the last of the stuffing. Press down lightly to make a smooth top.
Roll out the pastry sheet until wide enough to make a circular lid. Brush the overhanging edges with some of the second egg, beaten with a fork. Top with the pastry lid, pinch the edges to seal and then trim the excess. You can use the trimmings to decorate, if you like – just stick on with some more egg then give the whole top a good brush. Sit the tin on a baking sheet and bake for 50-60 minutes until well browned, then cool in the tin for 15 minutes before lifting from the tin sides and leaving to cool completely. Great as part of a buffet, eat it with salad leaves, chutney and more cranberry sauce.