In a small nonreactive saucepan, combine 1/4 cup water, strawberries, rhubarb and 3 tablespoons of the sugar and heat over medium-high heat. Let mixture simmer for 20 to 30 minutes or until thickened and fruit has broken down, like a thick jam. Remove from heat and let cool to room temperature. When cool, strain off any extra liquid. (Can be prepared in advance.)
In a small bowl, add remaining 3 tablespoons sugar and vanilla. Every so often, while preparing ingredients, stir with a small fork to combine. The sugar with absorb the vanilla.
Preheat the oven to 375 degrees F.
In a medium bowl, combine the rhubarb-strawberry compote with the cream cheese, sour cream, 1 egg, lemon zest and pistachios.
In a small bowl, beat the remaining egg and add 1 tablespoon of water.
Cover a large baking sheet with parchment paper or a silicone baking sheet. Working quickly with the dough, unfold the sheets. Cut each sheet into 4 equal squares. Place a scant 1/4 cup filling inside each square and wet the outer edges with egg wash. Fold the pastry over to form a triangle and seal with a fork. Brush with egg wash and sprinkle with the vanilla sugar.
Transfer to the prepared baking sheet and bake for 25 to 30 minutes. Let rest for 10 minutes before serving. Garnish top with powdered sugar and sliced strawberries.