In a medium sized pot, melt the butter and sweat the onion and celery (with the lid on) until its softened (but not browned). Set aside in a mixing bowl.
Rinse the quinoa in a fine sieve really well for at least 1 minute (this removes the bitter saponins from the quinoa). Cook the quinoa by placing it into a pot with 2 cups of water.
Bring the mixture to a boil over medium-high heat, then decrease the heat a bit to maintain a gentle simmer. Cook uncovered until the quinoa has absorbed all of the water, about 10 to 12 minutes.
Remove the pot from heat, cover, and let the quinoa steam for 5 minutes. This step gives the quinoa time to pop open and the little white spiral to appear. After 5 minutes, remove the lid and fluff the quinoa with a fork. Allow to cool.
Now mix all the stuffing ingredients together. Taste and adjust the seasoning before adding the egg and mixing through. Keep airtight in the fridge until ready to add to the turkey (I make this a day ahead).
I personally take the stuffing to my butcher to de-bone the turkey and roll it for me. If you want to tackle this yourself, there are heaps of YouTube tutorials on boning and rolling a whole turkey.
Once the turkey is boned, it will weigh approx. 4 kg. Work off around 30 mins per kg.
Pre-heat oven to 200°C / 400°F (fan-forced).