Prepare the filling: In a medium saucepan, combine the diced rhubarb, strawberries, granulated sugar, cornstarch, and lemon zest. Stir gently to coat.

Cook the filling: Heat the fruit mixture over medium heat, stirring frequently. Bring to a simmer and cook for about 8-10 minutes, until the rhubarb is slightly softened and the mixture has thickened. Remove from heat and let cool completely (refrigerate for faster cooling).

Prepare the pastry: Preheat oven to 400°F (200°C). Line a baking sheet with parchment paper. Unroll the pie crusts and cut each into 3 equal squares (6 squares total).

Assemble the turnovers: Place about 2 tablespoons of the cooled filling onto one half of each pastry square, leaving a 1/2-inch border around the edges. Fold the empty side over the filling to form a triangle shape, pressing the edges together to seal.

Finish and bake: Crimp the edges with a fork to ensure a good seal. Cut two small slits in the top of each turnover to allow steam to escape. Whisk the egg with milk/water to create an egg wash, brush over the tops, and sprinkle with coarse sugar if desired.

Bake for 20-25 minutes, or until the crusts are golden brown and puffy. Let cool on the baking sheet for 10 minutes before transferring to a wire rack to cool completely.