Special equipment: a pressure cooker or stockpot
For the stock: Preheat the oven to 375 degrees F. Lightly butter a roasting pan and add the turkey parts in a single layer. Roast until golden brown, 20 to 25 minutes. Cool completely in the pan.
Put the roasted turkey in a pressure cooker with the onion, carrot, celery, garlic, soy sauce and 8 cups water. Lock the lid in place, put over high heat, and bring the cooker to high pressure. Lower the heat to maintain high pressure and cook 15 minutes. Remove from the heat. Let the pressure dissipate naturally, about 2 minutes, or according to the manufacturer's instructions. Remove the lid and skim the stock.
Alternatively put the roasted turkey pieces, the onion, carrot, celery, garlic, soy sauce and 8 cups of water in a stockpot over high heat. Bring to a simmer, reduce the heat to low, and simmer, skimming occasionally, for 2 hours. You should end up with about 8 cups of stock.
Strain the stock through a fine sieve. The stock may be used immediately or cooled and refrigerated for at least 4 hours and up to 24 hours. If refrigerated, the fat will congeal on the top of the stock and should be removed before continuing.
For the gravy: Whisk together the cream and tapioca in a small bowl until smooth. Bring 2 2/3 cup of the turkey stock to a simmer in a small saucepan. Reserve the remaining stock for another use. Whisk in the tapioca mixture. Bring the gravy back to a simmer and cook, stirring occasionally, until thickened, about 4 minutes. Taste, and season with salt and pepper. Serve hot.