Start by placing two cups of blueberries with sugar in a small pot. Mix water and cornstarch in a cup, then add it to the berries. Heat over medium heat until the mixture thickens and berries start releasing their juices, about 5 minutes. Gently stir in the remaining blueberries and let it cool.
Grate the lemon zest first, then squeeze the lemons for juice. Strain the egg yolks into a small pot and mix with sugar and lemon juice. Cook on medium heat until it thickens, but don’t let it boil. Remove from heat and whisk in butter and zest, then chill in the refrigerator.
Bake a tart shell using your favorite recipe or a pre-made shell. Once it cools completely, pour in the lemon curd, spreading it evenly. Carefully top with the cooled blueberry compote, avoiding mixing it into the curd. Refrigerate until ready to serve.