Shell the fresh beans. Finely chop the onion and also finely dice the bacon. Sauté the onion and bacon in the oil until translucent. Add the beans and enough water to just cover them.
Bring to a boil over low heat, then cook over low heat for 2 – 3 hours (depending on the type of bean), salting the beans only after cooking (important!
Then remove half of the beans from the pot and mash, then return to the pot, boil again. Add the tagliatelle to the soup and cook until al dente.
Season the soup with salt and serve hot.