Preheat oven to 375F and have a muffin tin ready to go. No need to grease the muffin tin - I like to use silicone liners for easy cleaning, but this is not necessary.
On a lightly floured surface, roll out the pastry dough to approximately ⅛" thickness.
Using a 3.5-4" circular cutter, cut out 12 rounds of pastry dough. Gently press the dough rounds into the muffin tin, allowing the dough to create folds along the sides if it wants. Feel free to use excess dough to seal any holes or tears.
If you are adding any mix-ins to the tarts, add a tsp or two to the base of the tarts.
Pour the filling into the tart shells, distributing evenly.
Bake for 30-35 minutes, until the edges of the crust is golden in colour.
Allow to cool before eating. Enjoy :)