Cranberry or borlotti beans are kidney shaped with red streaks. Used in all kinds of Italian soups and stews, they're worth seeking out. If you can't find them, substitute navy or cannellini beans. If you like a little heat, go for the larger amount of pepper flakes. This soup is pure comfort.
Soak the beans: Soak the beans in water for at least 8 hours.
Make the soup: Heat 2 tablespoons of the olive oil in a soup pot over medium heat. Add the onion and cook, stirring occasionally, until just tender, about 7 minutes. Add the garlic, rosemary, pancetta, and crushed red pepper flakes and cook, stirring, another 2 to 3 minutes.
Drain the soaked beans and add them to the soup pot along with the chicken broth. Bring to a boil, lower to a simmer and cook, uncovered until the beans are pleasantly tender, about 1 hour, sometimes longer, depending in the beans; taste a bean to determine tenderness. With a slotted spoon, remove the soup's solids to a large, rimmed baking sheet; this will make it easier to find the rosemary and pancetta.
Discard the rosemary. When the pancetta is cool enough to handle, cut it into a small dice. Set aside about 1/4 of the beans in a bowl and toss those beans with the diced pancetta. Puree the remaining beans along with the remaining broth either back in the pot with a handheld immersion blender or in batches in a food processor or blender. As you puree, add 2 to 3 tablespoons of olive oil, a little at a time to create a creamy, emulsified soup.
To serve: Return the soup to the pot if need be and add the reserved beans and pancetta; add more broth to thin the soup if you like. Reheat briefly, season with pepper and — if needed — salt, and serve.