In a small saucepan over medium-low heat combine the rhubarb, sugar, and lemon. Cook for 5-10 minutes, or until rhubarb is tender.
Remove from the heat and add in the strawberries.
Let cool completely, and thicken up to a jam like consistency.
Preheat oven to 400 degrees.
Line a large, rimmed sheet pan with parchment. Reserve.
Whisk the egg and water together in a small bowl.
Sprinkle flour over a work surface.
Roll out puff pastry with a rolling pin to flatten the seams.
Cut each piece of puff pastry into 4 large squares.
Place equal parts of the strawberry rhubarb mixture into the center of each of the squares.
Brush egg wash over the edges of the square.
Fold into a triangle over the filling.
Using the tines of a fork, press to seal.
Place the turnovers onto the parchment, leaving at least2-inches of space in between them.
Brush the egg wash over the turnovers, and sprinkle with coarse sugar.
Bake for 15-18 minutes, or until golden brown.
Let cool for a few minutes, and enjoy!
Note: Store in the refrigerator for up to 5 days. To refresh, just pop them in a 350 degree oven for a few minutes to warm through.