Line two baking sheets with parchment paper. Cut the pastry, widthwise, in half. Set the half pieces on a lightly floured surface and gently roll each one into a 10” x 10” square (they don’t have to perfectly formed). Set one piece of pastry in the centre of each baking sheet.
Place the rhubarb, strawberries, ½ cup sugar, flour, lemon zest and spices in a bowl and toss to combine. Divide and spoon this mixture on the squares of puff pastry, leaving a 2” border of clean dough. Fold the clean edges of pastry up and partially over the filling, leaving the centre open. Brush the top of the pastry with egg wash. Chill each galette in the refrigerator 30 minutes.
Preheat the oven 375 degrees F. Bake one of the galette 30 to 35 minutes, until puffed and golden. Set on a baking rack to cool. Repeat with the other galette.
When ready to serve, dust each galette lightly with icing sugar. Slice and serve the galette with dollops or scoops of whipped cream or ice cream.
Note: If the strawberries are very large, cut them in half, lengthwise, before slicing.