In a small pot, combine 2 cups of fresh blueberries and sugar over medium heat. Stir constantly for about 3-4 minutes until the blueberries start to burst and release their juices. Mix a tablespoon of water with cornstarch and add it to the pot. Cook for an additional 2 minutes, or until the mixture thickens. Remove from heat and stir in the remaining blueberries for a fresh texture; set aside to cool.

Grate the zest from 2 lemons. In a saucepan, whisk egg yolks with granulated sugar and fresh lemon juice over medium heat. Continuously stir for about 5-7 minutes, or until the mixture thickens and coats the back of a spoon. Remove from heat and whisk in cold butter pieces along with the grated lemon zest until smooth. Transfer to a bowl and chill in the fridge until set.

If using a pre-made tart shell, ensure it has cooled completely. Pour the smooth, chilled lemon curd into the tart shell, spreading it evenly. Carefully spoon the cooled blueberry compote on top of the lemon curd, ensuring an even layer. Refrigerate the assembled tart for at least 2 hours to set properly before slicing and serving.