Combine the blueberries, sugar, lemon zest, 1 tablespoon butter, and 2 tablespoons water in a medium pot and bring to a boil over medium-high heat. Reduce heat the heat and simmer for 5 minutes, stirring occasionally, until berries start to break down and become thick.
When making the filling for these mini blueberry tarts, I like to burst a few of the berries while they are simmering to get the filling a little thicker. Do this with the back of a wooden spoon - take care that it won't burst on you!
In a small bowl, mix cornstarch with 2 teaspoons water. Add this slurry to the blueberry mixture and keep stirring till the mixture thickens.
Take pot off the heat and stir in the lemon juice.
Allow the filling to cool completely before filling pie shells. This will also help the mixture to thicken more.
Once the mini flower tart shells cool, fill each shell with about 1 tablespoon of the blueberry pie filling.
Top with a fresh blueberry and a small mint leave, if desired. R