Prepare the rhubarb:
Preheat your oven to 375°F (190°C). Place the diced rhubarb in a bowl and sprinkle with 1 tablespoon of sugar. Stir gently to coat the rhubarb in sugar and set aside for about 10 minutes to let it release its juices. If you like a sweeter filling, you can add a little more sugar.

Make the cream cheese filling:
In a separate bowl, beat together the softened cream cheese, powdered sugar, vanilla extract, lemon juice, and sour cream (if using). Mix until smooth and creamy. Set aside.

Prepare the puff pastry:
Roll out the puff pastry sheets on a lightly floured surface. Cut each sheet into 4 squares (or rectangles, depending on the size you prefer). You should get about 8 squares in total.

Assemble the pastries:
Place a spoonful of the cream cheese filling in the center of each pastry square. Be sure not to overfill. Then, top each cream cheese-filled square with a spoonful of the sugared rhubarb.

Fold and seal the pastries:
Carefully fold each pastry square into a triangular or rectangular shape, depending on your preference. Pinch the edges of the pastry to seal them. Alternatively, you can leave the pastry open and create a more rustic “tart” shape.

Apply the egg wash:
For a golden, glossy finish, lightly brush the tops of the pastries with the beaten egg mixed with 1 tablespoon of water. This step is optional but will enhance the look of the finished pastries.

Bake the pastries:
Place the pastries on a baking sheet lined with parchment paper or a silicone baking mat. Bake for 18-22 minutes, or until the pastries are golden brown and puffed up. Keep an eye on them toward the end to make sure they don’t burn.

Serve:
Once baked, remove the pastries from the oven and let them cool slightly before serving. These are best served warm, but they’re also delicious at room temperature. You can dust them with a little extra powdered sugar before serving for a finishing touch.