In a small saucepan, combine blueberries, sugar, and lemon juice.

Cook over medium heat until berries break down and sauce thickens (about 5 minutes).

Let cool slightly, then strain through a fine mesh sieve if you prefer a smooth sauce. Set aside.

Preheat oven to 350°F (177°C).

Mix melted butter, sugar, salt, and vanilla extract in a bowl. Add flour and combine until it forms a dough.

Press the dough evenly into a 9-inch tart pan. Prick the bottom with a fork to prevent puffing.

Pre-bake the crust for 10–12 minutes until lightly golden. Set aside to cool slightly.

In a medium bowl, whisk sweetened condensed milk, lemon juice, lemon zest, and egg yolk until smooth.

Pour the lemon filling into the pre-baked crust.

Drop small spoonfuls of blueberry sauce onto the filling. Use a toothpick to create swirls for a decorative effect.

Bake for 15–18 minutes, or until the center is just set (slightly jiggles).

Remove from oven and let cool to room temperature, then chill in the fridge for at least 2 hours before serving.