Combine all the masterstock ingredients except for the geese and bring to a soft boil. Reduce the heat to a low simmer and cook for 1½ hours. Remove from the heat and allow to cool completely. Strain the masterstock, discarding the solids.
Put the masterstock into the pot of a Gastrovac. Remove the legs and neck from the goose.
Put the goose breasts, on the crown (bone) into the masterstock, making sure both 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.
Remove the pot from the Gastrovac and immediately chill in the refrigerator, leaving the goose in the masterstock until required.
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