For the tagliatelle with tomato-mozzarella sauce cook tagliatelle. Sauté finely chopped onion in olive oil until translucent, add finely chopped tomatoes and simmer a bit.
Add tomato ketchup and simmer a bit more. Season with herbs, salt and pepper. Finally, add the mozzarella cheese cut into small pieces.
When the mozzarella has melted a bit, pour the sauce over the pasta.