In a medium saucepan over medium-low heat, mix the diced rhubarb and sugar. Stir occasionally and let it simmer until the rhubarb is tender, about 5 to 10 minutes. Remove from heat and gently mix in the strawberries and cinnamon. Set aside to cool, then strain any excess liquids.

Preheat your oven to 400°F and prepare two baking sheets by lining them with parchment paper.

Unfold the thawed puff pastry sheets on a lightly floured work surface. Use a rolling pin to gently roll them out, sealing any perforations. Cut each sheet into four equal squares.

Place a spoonful of the strawberry-rhubarb filling into the center of each pastry square. Fold them in half diagonally to form triangles, and use a fork to crimp the edges securely.

Create three small slits on top of each turnover. Brush with the whisked egg wash and sprinkle with sanding sugar if desired.

Bake on the prepared baking sheets for 15 to 18 minutes, or until golden brown and puffed up. Transfer to a cooling rack to cool before serving.