Clean the onion and chop it finely. Braise it in the olive oil until golden. Add the mince and stir until it changes color, stirring constantly. Next, add 3 tbsp tomato sauce. Season with salt and black pepper.
Fill the cannelloni with the resulting mixture.
Add the confectionery cream to the remaining tomato juice and stir until homogeneous.
Pour some of this sauce into the bottom of a suitable oven dish. Arrange the cannelloni and pour on the rest of the sauce.
Bake in a preheated oven until ready.
Before they're done baking, arrange thin slices of mozzarella on top of the cannelloni.
Bake until the mozzarella melts.