For the stuffing, melt the butter in a large frying pan and gently soften the sliced leeks for about 5 minutes. When squishy, stir in the sage and mace for another minute. Scoop from the pan into a big mixing bowl and leave to cool.
When the leek mixture cools, squeeze the sausage meat from the skins into the bowl too, then add the breadcrumbs, pecans, and cranberries with some seasoning. Mix everything together really well.
On a floured surface roll out the first block of pastry thinly, until it’s the size of an open magazine.
Lift the pastry onto a baking sheet, then arrange the turkey breasts on top, in a line down the middle. Press the stuffing evenly and firmly over the top and sides of the turkey. Then dot the sage leaves over the stuffing.
Roll out the second block of pastry on a floured surface so it’s big enough to cover. Brush the pastry border left around the turkey with some of the egg. Lay the second pastry sheet over the top, and press the edges together to stick. Trim to neaten, leaving a border of 2-3 cm, then brush all over with more egg before scoring a pattern in the pastry with a small knife. Bake immediately or chill for up to 48 hours.
Preheat the oven to 200°C (180°C fan oven) Gas Mark 6. Cook for 1 hour-1 hour 15 minutes, covering loosely with foil after 40 minutes if its getting too brown. After the hour, check the meat is cooked by inserting a metal skewer into the middle for 5 seconds – it should feel hot to the touch on your wrist when it comes out. Rest for 10 minutes, then slice and serve with cranberry sauce, gravy and your favourite trimmings.