Bring a large pot of well-salted water to a boil over medium heat and set up a bowl of well-salted ice water. Add the shelled fava beans to the boiling water. Once the water has come back to a boil, cook the beans for 3 to 4 minutes. Remove them from the boiling water and immediately put them into the bowl of salted ice water. Once the beans have cooled completely, strain them from the ice water. Peel the tough, light green outer layer from the beans. You will be left with a delicate, lovely vibrantly green tender fava bean. MMMMMM! Reserve these little lovelies.
Coat a large saucepan over medium-high heat with extra-virgin olive oil. Add the bacon (if using, if not just start with the onions and celery) and once the bacon has started to become brown and crispy and is very aromatic, add the diced onions and celery. Season with salt, to taste, and a pinch of crushed red pepper. Cook the onions and celery until they become translucent and are very aromatic. Add the smashed garlic cloves and the diced potato, stir to coat with the oil and cook for 2 to 3 minutes. Stir in 2/3 of the reserved fava beans and 4 cups stock. Taste for seasoning and add salt, if needed. Bring the stock to a boil, then reduce the heat and simmer until the potatoes are fork tender, about 20 to 30 minutes.
When the potatoes are tender enough to slide easily off a fork when cooked, puree the soup in a blender* until smooth. You probably will have to do this in batches. Return the soup to the pot, and add the remaining fava beans. If the soup is too thick, add some the remaining stock to adjust the consistency. Taste the soup to check the seasoning and reseason, if needed.
Ladle the soup into serving dishes, give them a little sprinkle of grated Parmesan and a drizzle of big fat finishing oil.
Fava beany-weany delicious!
When blending hot liquid, first let it cool for five minutes or so, then transfer it to a blender, filling only halfway. Put the lid on, leaving one corner open. Cover the lid with a kitchen towel to catch splatters, and pulse until smooth.