1. Peel the onion, shallot, carrot, and celery. Chop them roughly and blend them. Put this mixture in a large pan with a little olive oil, stir.
2. Crush the garlic cloves without peeling them. Add them to the vegetables.
3. Pour in the balsamic vinegar, tomatoes, and tomato paste. Mix and season to taste.
4. Cut the sausages into pieces, add them to the sauce, and let them cook gently.
5. Cook your pasta in a large pot of boiling salted water. Leave them 1 minute less than the time indicated on the package.
6. Take some pasta cooking water and pour it into the pan, this will help loosen the sauce.
7. Drain the spaghetti and pour them into the pan. Mix well and let them cook for another minute.
8. Serve the spaghetti with the pieces of sausages in individual plates.