In a large saucepan, sauté the chopped onion and garlic in a little olive oil.
Add the diced tomatoes and bring to a boil.
Add the herbs, reduce the heat and simmer uncovered for 45 minutes to 1 hour.
Blend the mixture with an immersion blender until smooth and homogeneous.
If the mixture seems too liquid, return it to the heat and cook until the desired consistency is reached.
Let it cool completely.

While preparing the sauce, prepare the polpettines:
Remove the skin from the sausages and shape the meat into small balls, about the size of a small nut.
Reserve the polpettines in a plate in the refrigerator until assembly.

Preheat the oven to 450 F (230 C).
Sprinkle cornmeal on 2 pizza pans.
Divide the herb sauce evenly between the two pizza doughs, leaving a 1-inch (2.5 cm) border.
Distribute the polpettines over the sauce and sprinkle with shredded mozzarella.
Bake in the lower third of the oven for 15 to 20 minutes or until the mozzarella starts to brown.
Remove from the oven and serve immediately.