Mix sugar, cornstarch, and salt in large microwave-safe bowl. Mix in water and rhubarb. Microwave on high, stirring occasionally, for about six minutes or until the rhubarb is tender and liquid has thickened to the consistency of jam. Cool to room temperature, about 30 minutes. Mix in strawberries and orange zest.
Preheat oven to 425°. Line two baking sheets with parchment or silpat.
Unfold pastry sheet on a lightly floured surface. Cut each pastry sheet in to 3 rows. Roll each row so it’s 12 inches long. Cut each row in to 3 squares. Roll each square to a 5-inch square. Turn the square so it’s on an angle and egg wash the two front edges. Put a spoonful of filling in the middle of the square and fold the top triangle over the bottom triangle. Seal the edges together with the tines of a fork.
Snip the top in three places to release steam.
Transfer turnovers to prepared baking sheets. Put sheets in the freezer for 10 minutes.
Remove from freezer and brush top of pastry with egg wash and a sprinkle of sugar if desired.
Bake pies 5 minutes. Reduce oven temperature to 350° and bake 25 minutes more until golden brown and puffy. Rotate the trays the last 10 minutes of cooking.
Whisk together powdered sugar, water, and vanilla until smooth. Add water a little at a time and add more powdered sugar or water as necessary. Drizzle glaze over hot turnovers.
*Thaw puff pastry sheets overnight in the refrigerator, or at room temperature for 40 minutes or until easy to handle.