The peppery arugula and good dose of parmesan cheese provide just the right amount of sharpness to this lovely white bean soup!
If you have time, put the beans in a large bowl and add enough cold water to cover by about 1 inch. Let the beans soak for at least 6 hours or overnight. Drain.
In a large soup pot heat the oil until just simmering. Add the onion, celelry, and garlic and cook for 12 to 14 minutes over medium-low heat, or until softened but not brown.
Add the beans, stock, and rosemary, and bring to a boil over medium-high heat. Reduce the heat to medium and simmer for about 50 to 60 minutes, or until the beans are tender.
Stire the arugula, cheese, salt and pepper into the soup and remove from the heat.
Transfer the soup, in batches, to a blender or food processor and process until very smooth. Return the pureed soup to the pot and season with salt and pepper. Reheat, if necessary and serve hot.