To make the crust:

Cup of Yum
1. In a large bowl, combine the flour, sugar, and salt.
2. Cut in the diced butter using a dough blade, your fingers, or forks until the mixture becomes coarse with no pieces larger than a pea.
3. Mix in the egg yolk and slowly add water until the dough comes together.
4. Form the dough into a disc, wrap in plastic, and refrigerate for 1 hour.

To make the filling:

1. Place the strawberries and blueberries in a large bowl.
2. Add the sugar, cornstarch, lemon zest and juice, vanilla extract, and salt. Toss until evenly coated.

To assemble:

1. Preheat oven to 400˚F (200˚C). Line a round, rimmed baking sheet with parchment.
2. On a lightly floured surface, roll the chilled dough into a thin circle about 14 inches (36 centimeters) wide.
3. Cut off the excess dough to make the circle into a pentagon shape with 5 sides.
4. Transfer the pentagon to the prepared baking sheet.
5. Pour the filling into the center of the dough and evenly spread it, keeping a 2 inch (5 centimeter) border around the edges.
6. Make a 1 inch (2.5 centimeters) slit in the center of each edge towards the middle.
7. Fold each edge over the filling, leaving the center of the filling open and creating a star shape. Pinch the overlapping edges of dough together to seal.
8. In a small bowl, beat together the egg yolk and water.
9. Brush the egg wash over the exposed areas of the dough.
10. Sprinkle the turbinado sugar over the dough.
11. Bake in the preheated oven until the crust is golden and the filling is bubbly, about 30 minutes.
12. Allow to cool for 10 minutes before cutting into slices and serving warm.