In a large bowl, combine flour and sugar. Cut shortening into flour mixture using a pastry cutter.
Combine 4 tablespoon of milk with 4 tablespoon water in a small bowl. Pour a bit at a time into large bowl and mix with each addition. You may not need to use all the milk/water mixture. Stop adding when pastry comes together. Divide pastry dough into two; wrap and refrigerate for 1 hour or up to 1 day.
Preheat oven to 350F. Grease a tart pan and set aside. In a small bowl mix mincemeat and sherry together. Set aside.
Roll out dough onto a lightly floured work surface, about ⅛" thick. Cut out fourteen 3" circles and line in tart pan. Fill cavities of tarts with mincemeat. Roll out fourteen 2.5" circle "lids." Using the 2 tablespoon milk, brush the edges of each tart and lay lids on top, crimping around the edges to seal. Cut slits in the top of each tart to create steam vents.
Whisk egg and 1 tablespoon water in a small bowl. Brush on top of each tart. Bake for 20-30 minutes, or until tarts are golden brown. Let cool for 10 minutes in tin and then remove to cool completely on wire rack. Dust top of tarts with icing sugar.
Store cooled tarts in an airtight container at room temperature for 3 days. Leftovers can be frozen up to 3 months. Thaw overnight in the refrigerator and bring to room temperature before serving.