Melt the butter gently in a saucepan over medium-low heat. Once melted, add the flour and stir briskly using a wooden spoon or whisk until a smooth paste forms. Let this roux cook for about 1 minute to eliminate any raw flour taste.

Gradually pour in half of the milk, whisking energetically to dissolve any lumps. Once smooth, add the remaining milk and continue to whisk until the sauce thickens into a rich, velvety texture.

Remove from the heat and immediately stir in the Parmigiano Reggiano cheese, a pinch of nutmeg, and season generously with salt and pepper. Transfer the finished white sauce into a container and set aside to cool completely.

Pat the mozzarella dry with kitchen paper to remove excess moisture, then cut it into small cubes approximately 1cm (0.5 inch) in size.

In a small bowl, mix the passata with a teaspoon of olive oil and season with salt and pepper. Stir well and set aside for assembling.

Once the white sauce has cooled, line a large baking sheet—or two smaller ones—with parchment paper.

Using a round cutter, cut out 7–8 discs from one sheet of puff pastry. Arrange the discs on the prepared tray, leaving a little space between each.

Spoon about 1 tablespoon of white sauce into the center of each disc, creating a shallow well in the middle. Add around 1 teaspoon of the tomato sauce into the well, followed by a cube of mozzarella.

Cut out matching pastry discs from the second sheet and carefully place them over the filled bases. Press the edges together firmly or crimp them to seal. Using the rim of a wine glass, lightly press the center of each pastry to indent without cutting through.

Transfer the tray to the refrigerator and chill the pastries for at least 1 hour to help them maintain their shape during baking.

Preheat the oven to 200°C (400°F). When ready, remove the chilled pastries from the fridge and lightly brush the tops with the beaten egg.

Bake for 15–20 minutes, or until the pastries are puffed and golden brown. Serve warm straight from the oven, or allow to cool and enjoy at room temperature.