Start by making a very thick béchamel following this recipe. Transfer the béchamel to a small bowl and let it cool completely. It should be very firm, more of a paste than a sauce, which is what you want.
Mix the passata with the olive oil and salt in another small bowl. If the passata is loose, reduce it by simmering it gently for a few minutes. It should coat a spoon thickly.
Beat the eggs in a third small bowl.
Lay the sheets of the puff pastry out on a cutting board. Using a pastry ring, cut out six rounds (or 12, if you want a double base).
Then cut out six rounds using a slighty smaller wide pastry ring from the third sheet. (See Notes below for recommended sizes.)
Lay out six of the larger rounds on a baking sheet lined with parchment paper. Brush them with beaten egg. If you like, then lay over another six of the larger rounds on top of them. Brush that second layer too with beaten egg.
In the middle of each round, place a spoonful of béchamel, then top it with a couple dice of mozzarella and a teaspoonful or two of the passata mixture. Make sure to leave a nice border around the edges free of any filling.
Now top each with the six smaller rounds of puff pastry. Press down lightly around the little mound in the center where your filling is, so the top adheres to the base, taking care not to press the edges of the larger rounds. Brush the tops, too, with beaten egg.
Now it's into a hot (200C/400F) oven for about 15-20 minutes, until the rounds have puffed up nicely and turn golden brown.
Serve at room temperature or just slightly warm.