Let the fava beans soak in water for at least 24 hours, 48 if possible.
Drain. Put 1 qt. of water in a pot with the fava beans, the onion and the peeled, seeded, chopped tomato. Salt, cover and cook over low heat for about 2 hours or until the beans begin to fall apart.
Break up the spaghetti into 1-in. pieces. Cook the pasta in plenty of salted water in a separate pot. When al dente, drain and add them to the fava beans. Add peperoncino and olive oil, mix well and serve.
This soup is also good at room temperature.
