Preheat your oven to 375°F (190°C) and line a baking sheet with parchment paper.

Chop the fresh rhubarb and toss it in a bowl with granulated sugar. Let it sit for about 10 minutes.

In a mixing bowl, combine softened cream cheese, powdered sugar, vanilla extract, lemon juice, and optional sour cream. Mix until smooth.

Roll out the puff pastry on a floured surface and cut into 8 equal squares, about 4 inches each.

Spoon about 1 tablespoon of the cream cheese mixture into each square, then top with rhubarb mixture. Fold and seal the pastries.

Whisk together egg and water for the egg wash. Brush it on the tops of the pastries.

Bake for 18–22 minutes until golden brown and puffed up.

Let cool for a few minutes on a wire rack. Optionally dust with powdered sugar before serving.