Heat up some oil in a skillet. Peel and cut the red onion into small cubes. Add them to the oil and fry them on medium heat until they are glazed.
Peel and chop the garlic. Turn the heat to low. Add it to the onion and fry for about a minute.
Add the tomato puree and stir well.
Optional: Chop the green chilli into pieces and add it to the sauce.
Add the mozzarella and stir until it combines with the tomatoes. Add salt and pepper to taste.
Cook everything on low heat for about five minutes until the sauce thickens.