The “recipe” is pretty fluid: for 4 cups of berries, start with a scant cup of white sugar, 3 tablespoons of corn starch, and a 1/4 cup of water. Stir the sugar and cornstarch together, then add the water. Add just enough water to be able to melt the sugar and cornstarch together. Pour over the berries in a medium saucepan and cook over low heat, stirring often, until berries release their juice, and mixture is hot and bubbly and has thickened. Taste at this point and add a bit of extra sugar if necessary. A teaspoon or two of lemon juice helps cut a too-sweet flavor. Let the berries cool about 40 minutes, stirring gently from time to time, then fill tart shells. For 3″ tarts, use about 1/8 cup of filling per tart. A batch of filling this size should fill about 12-15 tarts. Bake tarts at 375°F for about 14 minutes until pastry is golden and filling is bubbly. Remove from oven and cool before serving.