Put the masterstock in the pot of a Gastrovac. Remove the legs and necks from the ducks and reserve.
Put the duck breasts, on the crown (bone) into the masterstock, making sure they are fully submerged. Place the pot onto the Gastrovac and set it to 60°C (140°F) on full vacuum for a period of 90 minutes. The Gastrovac method means that you can cook under a vacuum so that the liquid will boil and cook the protein at a lower temperature.
Remove the pot from the Gastrovac and immediately chill in the refrigerator, leaving the duck in the masterstock until required.
Once the duck has cooled in the refrigerator, cut the breasts from the bone and reserve the breasts in the refrigerator. Sauté the remaining carcasses, including the legs, in a heavy-based saucepan with the grapeseed oil, onion and carrot. Once golden brown, add the chicken stock and simmer for 2 hours on low heat. Strain, discarding the solids, and return the liquid to the boil and reduce to a thick glaze. Set aside until required.
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