Make the Blackberry Filling

1. Place the blackberries, salt, sugar, lemon juice, and tapioca flour in a heavy-bottomed saucepan on stove over medium-high heat.
2. Stir to mix ingredients, then mash berries with a wooden spoon or potato masher to help release berry juice. Cook, stirring frequently, until mixture just comes to a boil.
3. Remove pan from stove. Stir in butter until it melts.
4. Cool blackberry filling completely in refrigerator before making turnovers. Filling will thicken considerably as it cools.

Make the Turnovers

1. Thaw puff pastry (see post for thawing tips).
2. After pastry is thawed, preheat oven to 400 degrees. Line rimmed baking pan (one or two, depending on size) with baking parchment.
3. Roll a sheet of pastry out on floured surface into a 12-inch square. Cut pastry into four 6-inch squares. Whisk egg and water together, to make an egg wash.
4. Place approximately 3 T of blackberry filling onto the center of each square. Use fingers to spread a half-inch line of egg wash around edges of one pastry square.
5. Fold pastry over filling and into a triangle, matching edges. Press fork tines firmly along the pastry edges, paying special attention to the corners. (See post for tips). Repeat steps 4 and 5 with remaining squares. The roll out remaining pastry sheet to make four more turnovers.
6. Place turnovers on prepared baking pan. Brush top of each turnover with egg wash. Bake in preheated oven for 20 minutes, until turnovers have puffed nicely and turned golden brown.
7. Remove pan(s) from oven. Cool turnovers completely before enjoying.