In a bowl, combine the spinach, Parmesan cheese, ricotta, egg, salt, pepper, and oregano. Mix everything until there is a compact mass.
Fill each sheet with 2-3 tablespoons of the mixture, always leaving a centimeter from each of the edges unfilled to make it easier to roll up later. Once rolled, cut each roll in half.
On a baking tray, put some baking paper and place each roll, always with the opening facing down so that nothing of the mixture comes out. Put a little tomato sauce on top of each mini-cannelloni and sprinkle with grated mozzarella.
Bake for 18 to 20 minutes, until the mozzarella has completely melted and is browned.
Then, prick them with chopsticks and serve them accompanied by tomato sauce.