Melt the butter in a pan, add the leek and a splash of water and cook for 5 minutes until softened. Set aside and allow to cool.
Roughly chop the lemon into pea-sized chunks. Leave the skin on, but discard any large pieces of pith. You'll need about 2 tablespoons-worth. Repeat with the orange.
In a bowl, combine the leek, citrus pieces, sausage meat, breadcrumbs and thyme. Divide the mixture in half and set aside one half for stuffing the neck of the bird. Lightly wet your hands and roll the remaining stuffing into 8 balls, each about the size of a golf ball. Wrap a piece of pancetta around each one.
Preheat the oven to gas 7, 220°C, fan 200°C, and bake for 25 minutes or until cooked through.
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For top tips on protecting you and your family when preparing raw meat and poultry, visit Food Safety in the Home and view the full Food safety at home video.