Preheat your oven to 350°F (180°C) and line a baking sheet with parchment paper. Unroll the cold puff pastry sheet onto the prepared baking sheet. Using a cookie cutter in the shape of a Christmas tree, cut out as many trees as possible. Carefully remove the excess dough, and space the pastry trees slightly apart on the baking sheet to ensure even cooking.
Prepare the pizza sauce by mixing the plain tomato passata with salt, extra virgin olive oil, and dried oregano (if desired) in a small bowl. Taste the sauce and adjust the seasonings if necessary to achieve a balanced flavor.
Using a spoon, spread a thin layer of the prepared tomato sauce over each puff pastry tree. Tear or shred the mozzarella ball into small pieces and scatter them evenly over the sauce-topped trees.
Place the baking sheet on the lower rack of the preheated oven and bake for approximately 15 minutes, or until the puff pastry is golden and the cheese is melted and bubbly.
Remove the trees from the oven, drizzle with a little olive oil, and sprinkle with a pinch of salt for added flavor. Serve immediately for a festive and delicious appetizer or snack.