Preheat oven to 200°C (400F) and line a large baking sheet with parchment paper.
Wash, hull and dice strawberries. Place in a small saucepan and add brown sugar, vanilla extract and lemon juice.
Simmer for 5-7 minutes until strawberries are softened and syrup is released. Use a fork or potato masher to lightly "mash" the strawberries. Stir in cornstarch mixture. Remove from heat, transfer to a glass bowl and allow to cool for 10 minutes.
Carefully unfold puff pastry and slice into 6 equal-sized rectangles.
Place 2-3 teaspoons of cooled strawberry filling in the center of each piece of puff pastry. Do not overfill the pastry or the filling will leak out.
Fill a small bowl with water. Dip your finger in water and run through the edges of each puff pastry and fold the top half over and seal the edges by pressing them using a fork.
Score the top of each turnover by making 3 small cuts then brush the surface evenly with egg wash using a pastry brush (lighty beat an egg and mix in 1 tsp. of milk). Sprinkle the top of each turnover with some brown sugar if desired.
Bake for 18-22 minutes until tops are golden brown. Remove from oven, cool for 5-10 minutes, sprinkle with powdered sugar, and serve.