In a mixing bowl, combine the flour and salt. Mix well. Add the butter and the lard and mix until the mixture resembles coarse crumbs. Add the egg and the water and let sit for 1 minute. Either using a fork or your hands, carefully press the mixture together to form a soft ball. Wrap in plastic wrap and refrigerate for at least 30 minutes. Remove the dough from the refrigerator and place it on a lightly floured surface. Roll out the dough, on a floured surface, into a rectangle 25 to 20 inches in diameter and 1/8-inch thick. Gently fold the dough in half and then in half again so that you can lift it without tearing it, and unfold into a 12 by 3 1/2 by 2 1/2-inch earthenware terrine mold.