To make the water pastry, mis the flour, diced butter and salt until crumble like. Add the cold water and beaten egg and knead until smooth and there are no more lumps Leave the pastry to set, and then line pie or terrine mould, leaving a piece for the lid.
For the Christmas terrine mix, slowly cook the onion, garlic, and sage leaves until softened. Mix with all the other ingredients in a mixing bowl and put inside terrine or pie dish.
Roll remaining pastry to fit the lid, cutting holes to release steam from. Keep trimmings to cut out festive garnish pieces if you wish.