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.