Make the sauce. If using plain tomato puree or tomato passata, cook it down in a small saucepan with a drizzle of extra virgin olive oil until it's no longer watery. Let cool completely. Add salt (and a pinch of sugar if the sauce lacks natural sweetness). Season generously with oregano.Alternatively, use precooked tomato sauce but watch out for it to be thick and not watery! If needed, you can leave it over a sieve lined with a paper towel to drain for 5-10 minutes.
Preheat the oven to 356°F (180°C) with fan ON or 375°F (190°C) fan OFF.
Roll and cut the dough. Roll puff pastry to about 1.5-2 millimeters thick (around 1/16 of an inch). Cut out 10-12 centimeters discs using a cutter or a glass. You'll need two discs for each pizzetta: one for the base, one for the lid.
Prepare the basis. Arrange half of the discs on a baking sheet lined with parchment paper. Brush with beaten egg.
Add the filling. Place a generous teaspoon of tomato sauce on each disc. Make sure to leave at least a 1-centimeter border for sealing. Top with 2-3 capers. (If making smaller size pizzette, 1 caper is enough).
Seal. Cover each base with the second disc and press around the edges with your fingertips or using a fork.
Glaze and bake. Brush each pizzetta with the remaining eggwash. Bake pizzette in a preheated 356°F (180°C) oven for about 15 minutes (+/- 2 minutes). Turn the baking sheet halfway through the time for even baking.
Let cool slightly and serve warm or at room temperature.