Preheat the oven to 350 degrees (F). Line a baking sheet with parchment paper.

Arrange the tart shells on the baking sheet and bake for 5 to 10 minutes, or until golden. Remove the shells from the oven and place on a wire rack to cool.

Next, pour 1-1/2 cups of the blueberries into a small saucepan. Do not turn on the heat on the stove yet.

In a small bowl, stir together the sugar, cornstarch, and salt. Add the sugar mixture and the lemon juice to the berries in the saucepan and stir. Next, bring the blueberry mixture to a simmer over medium heat and cook for 2 to 3 minutes, or until the berries burst and soften, and the juices thicken. Then allow the mixture to come to a boil, and cook for 1 minute more to thicken it further. This also removes some of the "starchiness" of the cornstarch.

Remove the saucepan from the heat and stir in the remaining 1-1/2 cups blueberries. Let the blueberry filling sit for 1 minute, and then spoon it into the reserved tart shells. Allow the tarts to cool for 10 to 20 minutes so that the filling firms up.

Serve the tarts with whipped cream, if desired. Refrigerate any leftover tarts.