Preheat the oven to Gas Mark 6, 200ºC, fan 180ºC. Slice off the top of the garlic head and place each head on a piece of foil. Spoon over the olive oil and season. Scrunch up the foil around it, leaving it loose.
In a large roasting tin, arrange the turkey pieces, onion, celery, carrots and the garlic wrapped in foil.
Season well and roast for 1½ hours or until browned. Using a slotted spoon, transfer the turkey and the vegetables into a large saucepan or stockpot. Pour the dripping into a measuring jug and chill for later. Set aside the garlic to use later when whisking the gravy.
Pour enough water into the stockpot to cover everything by 4cm (2in). Add the bay leaves and a few whole peppercorns. Place a lid on to partially cover it. Bring to the boil and then turn down to a simmer and leave for 1½ hours. Don’t be tempted to stir while cooking as this will make the stock cloudy.
Add 1 teaspoon salt and then pour through a fine mesh sieve. Discard the vegetables but keep the turkey leg meat for sandwiches. Measure out 900ml (1½ pints) of the stock for the gravy and freeze the rest for other uses.
Heat a heavy saucepan. Take the measuring jug with the original dripping from the fridge (the fat and juices will have separated), then add 3 tablespoons of the fat to the saucepan and heat. Discard the remaining fat from the measuring jug but keep the juices to one side.
Fry the shallots and thyme in the pan until softened. Add the flour and whisk for 2-3 minutes until very smooth. Keep the heat low so it doesn’t burn. Squeeze in the roasted garlic and add the lemon juice. Pour in the juices and stock and continue whisking for 5-8 minutes until thickened. Season to taste with salt and pepper.
Freezing and defrosting guidelines
Make the gravy, then leave to cool at room temperature. Freeze in a rigid container, leaving a bit of space for expansion, for up to 1-3 months. Reheat until piping hot either from frozen or defrost in the fridge overnight.
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