Preheat your oven to 400°F (200°C) and line a baking sheet with parchment paper.

Make the pesto: combine walnuts, salt, and garlic in a food processor. Pulse until it resembles coarse sand, then add basil and lemon juice. Drizzle in olive oil until smooth, and fold in Parmesan.

Prepare the pastry by rolling out the thawed puff pastry on a floured surface, cutting into 9 equal squares.

Assemble the pastries with a dollop of pesto, a slice of Roma tomato, and mozzarella on each square. Fold corners to enclose filling.

Chill assembled pastries in the refrigerator for about 20 minutes.

Brush pastries with beaten egg wash and sprinkle with garlic salt and Parmesan before baking.

Bake for 20 to 25 minutes until golden brown and puffed.

Cool briefly, then drizzle with balsamic glaze and garnish with basil leaves before serving.