Preheat the oven to 375F. Put the tart shells on a baking sheet and slide them into the oven for 10-12 minutes, until golden. Take them out.

Meanwhile, put half the blueberries into a medium saucepan. In a small bowl, stir together the sugar, cornstarch and salt. Add to the berries along with the lemon juice and bring to a simmer; cook for a few minutes, until the berries burst and soften and the juices thicken. Once it comes to a boil, make sure it cooks for a full minute to maximize thickening potential and make sure it's not starchy.)

Remove from the heat and stir in the remaining blueberries. Let sit for a minute to cool slightly, then spoon into the tart shells, piling them high.