Crostata

Ingredients

• 1/2 cup unsalted butter (cubed)
• 1 1/4 cups all-purpose flour
• 2 tablespoons cornstarch
• 1 large egg
• 1 tablespoon superfine sugar
• 1 cup blueberries
• 1 1/2 cups raspberries
• 1 1/2 cups strawberries (trimmed and sliced)
• 1 tablespoon lemon juice
• 3 tablespoons water (divided)
• 1/4 cup sugar