Make the shortcrust pastry according to instructions and chill before starting on the tarts.
Preheat the oven to 400°F (200°C) and set out a 12-cup muffin tin, plus a second tin if you have one.
Divide the finished dough in half, then roll it out on a lightly floured surface to 3-4mm (⅛ inch).
Cut circles of dough slightly larger than your muffin cups in diameter (about 9 cm or 3.5 inches is good). Set aside. Using a slightly smaller cutter (7 cm or 2 ¾ inches) cut tops for the tarts. You can also cut out small shapes (stars, hearts, etc.).
Place the larger rounds into the muffin cups, pressing gently to make sure they’re touching the bottoms of the cups. Fill each tart about ¾ full with mincemeat (about 2 tablespoons each).
Top each tart with a smaller round or shape cut from the pastry. If using a full round, dampen the edges of the lid with a touch of water to help it adhere to the base.
Use a fork to poke holes in the tops of any full pastry lids that don’t have shapes cut out.
Beat the egg with a teaspoon of water and use a pastry brush to brush the tops of each tart with egg wash.
Bake the tarts for 20-22 minutes, or until golden. Cool in the tin for 10 minutes before carefully removing and cooling fully. Dust with icing sugar when cooled.