Preheat the oven to 375F. Place the tart shells on a baking sheet and bake for 10-12 minutes, or until golden.
Heat half of the berries with 1/4 cup water in a medium saucepan over medium-high heat. Cook for a few minutes, until the berries soften and burst. In a small bowl, stir together the golden yellow sugar and cornstarch (use 1 Tbsp for saskatoons, 2 Tbsp for blueberries, which tend to be juicier) and stir into the berries along with the lemon juice and a pinch of salt.
Bring to a boil and simmer for 1-2 minutes, until the mixture has thickened and gone from slightly opaque to clear. Stir in the fresh berries and remove from the heat. Spoon the filling into the tart shells, piling them higher as the berries cool and the mixture thickens. Immediately before serving, dust the tarts with icing sugar, if you like. Makes 1 dozen tarts.