Preheat oven to 400 degrees F.
In a mixing bowl or the bowl of a stand mixer, combine cream cheese, the juice of the lemon half and sugar. Beat to combine, about 1 minute.
Spread out one sheet of thawed puff pastry on a parchment-lined baking sheet. Slather the cream cheese mixture on the puff pastry.
Slice guava into quarter-inch pieces. Cover the cream cheese-topped puff pastry with pieces of guava paste. Top with the other sheet of puff pastry, creating what looks like a massive puff pastry sandwich. Cut into 12 squares. Score the tops of each square with an X (or any design of your choice).
In a small bowl, crack an egg and beat it until pale yellow. Brush the top of the puff pastries with the egg wash. Bake for 20 minutes, or until the tops are golden brown.
