Prepare the pastry: In a large bowl, mix flour, sugar, and salt.

Cut in cool butter until the mixture resembles small peas.

In a separate bowl, whisk egg yolk, lemon juice, and water.

Sprinkle the egg mixture over the flour mixture and stir with a fork. Add more water if needed to form a dough.

Shape the dough into a ball using your hands.

Press the dough into a flan pan to 1/8 inch thickness.

Chill the pastry while making the filling.

Preheat oven to 425F (220C).

Prepare the filling: In a saucepan, combine water and cornstarch until smooth.

Stir in sugar.

Add boysenberries and cook over medium-low heat for 10-15 minutes, or until thickened.

Let the filling cool slightly.

Spoon the filling into the chilled pastry shell, filling no more than 2/3 full.

Bake in the preheated oven for 10 minutes.

Reduce the heat to 350F (175C) and bake for another 15 minutes, or until the pastry is golden brown.

Let the tart cool in the flan ring for 15 minutes before removing it to a rack to cool completely.