Preheat the oven to 400°F (200°C) and prepare a baking sheet lined with parchment paper.

In a blender or food processor, combine walnuts, salt, and garlic. Pulse until a sandy mixture forms, then add basil and lemon juice. Blend until combined, and slowly drizzle in olive oil until smooth. Fold in Parmesan cheese.

On a floured surface, roll out the puff pastry to smooth it out. Cut into nine equal squares.

Spoon a dollop of pesto onto each square, then add a slice of tomato and mozzarella. Fold corners towards the center and pinch to enclose the filling.

Chill the assembled pastries in the fridge for at least 20 minutes to maintain their shape during baking.

Remove pastries from the fridge, brush tops with egg wash, and sprinkle garlic salt and additional cheese.

Bake in preheated oven for 20-25 minutes or until puffed and golden brown.

Allow pastries to cool slightly, drizzle with balsamic glaze, and garnish with small basil leaves before serving.