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For the dough:
170 g margarine (for baking)
6 tablespoons very cold water
1 teaspoon salt
300 g flour (about 2 cups)
For the filling:
500 g blueberries (fresh or frozen)
3 tablespoons cornstarch
120 g sugar
Juice from 1/2 lemon (optional)
In a bowl, combine the flour, margarine (cut into small pieces), and salt. Mix with your hands until the dough becomes crumbly.
Add the cold water and continue mixing until the dough pulls away from the sides of the bowl. The dough should be smooth and workable, though it may seem crumbly or tough at first.
Chill the dough in the fridge for 30 minutes.
Wash the blueberries well, then mix them in a bowl with the lemon juice (if using), sugar, and cornstarch.
Preheat the oven to 200°C (392°F).
Remove the dough from the fridge and divide it into two pieces.
Flour your work surface and roll out the first piece of dough into a circle. Don’t worry if the edges crack — you can easily reshape it with your hands or rolling pin.
Carefully transfer the rolled dough into the tart pan, pressing it gently into the edges.
Pour the blueberry mixture onto the dough in the tart pan.
Roll out the second piece of dough and place it over the blueberries. Seal the edges of the dough by pinching them together, and make a few slashes in the center with a knife to allow steam to escape.
Place the tart in the oven and bake for about 45 minutes, or until the crust is golden and slightly browned on top.
Let the tart cool before slicing and serving. It can be served warm, but note that the syrup from the berries will spill out if still hot. If you let it cool completely, the slices will hold together better.