Prepare the Tart Crust: If using a pre-made tart crust, follow package instructions. For homemade, roll out the pastry and fit it into a 9-inch tart pan. Prick the bottom with a fork and chill for 30 minutes.

Pre-bake the Crust: Preheat the oven to 375°F (190°C). Line the tart shell with parchment paper and fill it with pie weights or dried beans. Bake for 15 minutes, then remove the weights and parchment. Bake for another 10 minutes or until lightly golden. Let it cool.

Make the Filling: In a mixing bowl, whisk together the sugar, eggs, and egg yolks until smooth. Stir in the heavy cream, lemon juice, lemon zest, and salt (and cornstarch if using). Mix until fully combined.

Bake the Tart: Pour the lemon filling into the cooled tart shell. Bake at 350°F (175°C) for 25 to 30 minutes or until the filling is set but slightly jiggly in the center. Let it cool completely on a wire rack.

Combine Ingredients: In a saucepan over medium heat, combine the blueberries, sugar, and lemon juice. Stir occasionally until the berries start to break down and release their juices.

Thicken the Sauce: If using, mix the cornstarch with water to create a slurry. Gradually stir it into the blueberry mixture, cooking for an additional minute or until the sauce thickens. Remove from heat and let it cool.

Once the lemon tart has cooled completely, slice it into wedges. Drizzle the blueberry sauce over the individual slices or serve it in a small bowl on the side. Garnish with fresh blueberries or a sprig of mint for a decorative touch.