Place the turkey, breast side down, on a cutting board
Use a knife to score down either side of the backbone. Use kitchen scissors to cut out the backbone completely. Turn the turkey over and press down firmly on the breastbone to flatten. You should hear a crack and feel the bones give way.
Preheat the oven to 160 degrees celsius, gas mark 3. Arrange the onions, carrots, celery, garlic and thyme in a roasting pan. Drizzle with the olive oil. Place the flattened turkey on top of the vegetables.
Bring the vinegar, soya sauce, brown sugar and spices to a boil in a small saucepan over a medium heat, stirring to dissolve the sugar.
Reduce the heat and simmer, stirring occasionally, until the mixture is thickened to the consistency of molasses (it needs to be very thick to stick to the turkey), approximately 5-8 minutes. Strain and allow the glaze to cool completely.
Brush the turkey with the glaze, add the water to the pan and roast the turkey for 50 minutes, basting with the glaze every 20 minutes until the skin is a deep golden brown and crisp. Check if the turkey is done by piercing the thigh, if the juices run clear, the turkey is cooked.
Cover with the foil and allow to rest for 30 minutes before carving. Garnish with the fresh thyme leaves.