1. Combine rhubarb, granulated sugar, cornstarch, and salt in a small saucepan and let sit until rhubarb has softened and released some of its liquid, about 30 minutes, stirring occasionally. Bring to a boil over medium-high heat and cook until rhubarb has broken down, mixture is very thick, and a film has formed on bottom of pan, about 5 minutes.
2. Remove from heat, add strawberries, and stir until fully coated with rhubarb mixture. Transfer to a medium bowl and let cool completely, about 1 hour.
3. Center a rack in oven and heat to 400 degrees. Line a baking sheet with parchment paper. Whisk egg and water in a small bowl until combined and set aside.
4. Working on a lightly floured surface, roll one sheet of puff pastry into a 9x12" rectangle, then cut into nine 3x4" rectangles; repeat with second sheet. Spoon 1 heaping tablespoon filling into center of nine rectangles, leaving a 1" border.
5. Brush edges with egg mixture and top with remaining nine dough pieces. Press edges down with fingers, then crimp with a fork to seal. Cut three 1/2" slits in center of each pastry. Transfer pastries to prepared baking sheet and freeze for 15 minutes. (If baking sheet doesn't fit in freezer, freeze pastries on two plates, then transfer to baking sheet after chilling.)
6. Brush pastry tops with remaining egg mixture, then transfer to oven and bake golden brown, 20 to 25 minutes, rotating baking sheet halfway through. Meanwhile, whisk confectioners' sugar, cream cheese, and lemon juice in a bowl until smooth (mixture will be very thick at first) to make an icing. Transfer to a resealable sandwich bag, press out air, and seal.
7. Use a spatula to transfer pastries to a wire rack. Snip a corner off icing bag and drizzle over warm pastries. Let cool 15 minutes before serving.
Source: Hannaford fresh Magazine, May - June 2019