Preheat the oven to 400°F.
Pour the store-bought mincemeat into a large mixing bowl. Add the diced apple, dried apricot, diced orange segments, lemon and orange zest, and the nutmeg. Gently stir until well mixed.
Roll the pastry out thinly on a lightly floured surface, then use a circular 3½-inch cutter to cut out pastry circles. Line two muffin pans with the pastry, making sure to get it into the corners of the pan.
Re-roll the scraps and cut out 15 stars with a cookie cutter to act as a lid for half of the tarts. Leave the other half plain, if desired.
Fill each tart ¾ full. Top each one with a pastry star, pressing down its corners to seal it to the tart. Sprinkle the stars with caster sugar.
Bake the tarts for about 20 minutes at 400°F. The filling should be bubbling, and the pastry should be a pale golden brown around the edges. If you're re-using a muffin pan to bake more tarts, let it cool before putting the next batch of pastry in the pan.
Let the tarts cool for 10 minutes in the pan, then run a knife around the edges of each tart and carefully pop them out of the pan.
Dust the star tarts with icing sugar, then serve.