Preheat oven to 200°C conventional oven/ 180°C fan-forced (400° Fahrenheit/Gas 6).
Cut 2 sheets of baking paper (the same size as 2 baking trays). Place the puff pastry sheets on top of the baking paper. Using a 6cm round biscuit cutter, cut 21 rounds out of the puff pastry. Use a fork to lightly beat the egg into a small bowl. Brush the puff pastry rounds with the beaten egg.
Place a teaspoon of Italian passata in the centre of each puff pastry round. Evenly distribute the mozzarella in the centre of each round (approximately 6 cubes per round). Place the Kalamata slices on half of the pizzette di sfoglia (puff pastry mini pizzas). Sprinkle the remaining pizzette (without olives) with dried oregano.
Transfer the 2 sheets of baking paper with pizzette di sfoglia to baking trays. Bake pizzette di sfoglia for 12 -15 minutes. Serve immediately or bake in advance and reheat for 3 minutes before serving – buon appetito.
3 sheets frozen puff pastry (approximately 500g), just thawed|1 egg|100g mozzarella, cut into 1cm cubes|70g Italian passata|30g Kalamata olives, pitted and thinly sliced|½ teaspoon dried oregano