Mixed Berry Crostata

This recipe is still a work in progress. The crust was delicious,buttery and flaky, but the berry filling was not sweet enough. Next time, I need to add more sugar. Also, I added the cornstarch directly to the berry mixture instead of first dissolving it in water, which gave it a lumpy texture. Fortunately, my husband was able to rescue it from sure disaster. Here is the recipe as it stands now, and I'll provide an update later.

Dough:
1 large egg
1 tsp. vanilla extract
1 tbsp water
1 1/3 cup all-purpose flour
1/3 cup sugar
1/4 tsp salt
1 1/4 tsp. baking powder
grated zest of 1 lemon
7 tbsp. cold unsalted butter, cut into 1/4 inch pieces

Filling:
1 lb. bag of frozen mixed berries
juice of 1/2 lemon
1 tbsp cornstarch
about 1/4 cup water (for dissolving the cornstarch)
1/4 cup sugar (try 1/2 cup for a sweeter filling)

Directions:
To make the dough:
Whisk the egg, vanilla, and water in a small bowl and set aside. Combine in the food processor the flour, sugar, salt, baking powder, and lemon zest. Add the butter pieces and mix until the mixture resembles coarse meal. Add the egg mixture with the food processor still running and mix to form the dough. Remove the dough from the food processor, wrap in plastic wrap, and refrigerate for about 2 hours.

For the filling:
Heat the frozen berries in a medium saucepan. Add the juice of half a lemon. Dissolve the cornstarch into a small amount of water and add to the berries. Add the sugar and stir continuously until the mixture thickens. Take off heat and cool for about 30 minutes.

To construct the crostata:
Preheat the oven to 375 degrees. Take the dough out of the refrigerator and knead until it is malleable. Roll out the dough into a circle of about 1/4 inch thickness. Transfer the dough to a greased baking sheet. Spread the berry mixture into the center of the circle, leaving about a 2-inch border. Fold the border over the filling. Bake for about 30-35 minutes, or until the crust is golden brown.Cool before serving.

Source:
Portions of this recipe are taken from: Baking Illustrated: A Best Recipe Classic Brookline, Mass.: America's Test Kitchen, 2004, page 236-238.