2. Drain beans, put them in a pot and add the stock, carrot, celery and enough water to cover the beans by an inch or so.
3. Bring to a simmer, uncovered, over medium-high heat. Do not boil. Skim foam that comes to the surface. (Do not add salt because it will prevent the beans from becoming tender.)
4. When the beans are tender, about 1 hour, remove from heat. Drain and discard the carrot and celery. Set beans aside. (The cannellini beans can be cooked ahead of time, covered and refrigerated up to 3 days.)
5. In a large skillet, heat olive oil over medium heat. Add the garlic and cook for about 1 minute. Add the beans, season with salt and pepper, and heat thoroughly.
6. Turn off the heat and gently fold in the arugula leaves. Serve warm or at room temperature.