Preheat the oven to 400°F and line two baking sheets with parchment paper or Silpat baking mats.
In the bowl of a stand mixer fitted with the paddle attachment, combine the cream cheese, lemon juice, lemon zest, vanilla extract and sugar. Beat until creamy and well combined. Transfer the mixture to a sealable plastic bag or piping bag (optional).
Lightly flour your work surface then unfold each sheet of puff pastry and roll it into a 10-inch square. Cut each square into four smaller squares. To make diamond shapes, fold the squares in half diagonally to make triangles then cut a 1-inch border around the triangles except for the very tip. Open the pastries then invert the folds of dough so they’re opposite of one another. (See photos above.)
Arrange the pastries on the lined baking sheets, spacing them 2 inches apart. Pipe a portion of the cream cheese filling into the center of each pastry. Top the cream cheese with the sliced strawberries.
In a small bowl, whisk the egg. Using a brush, generously brush the pastries all over with the egg wash, omitting the area with the cream cheese. Sprinkle the edges of the pastries with the sanding sugar (optional).
Bake the pastries for 15 to 18 minutes or until golden brown and puffed. Remove them from the oven then transfer them to a rack to cool before serving.