Heat the oil in a large saucepan over medium heat. Add the celery, carrot and onion and cook, stirring, for 5-6 minutes until the vegetables are tender and begin to brown. Add the garlic and hot pepper flakes and cook 1 minute longer. Add the potato and 1 cup of stock. Bring to a boil, turn down to a simmer, cover and cook about 10 minutes.
Drain and rinse the beans. Place half of the beans in a food processor with 1 cup of stock and purée. Stir into the saucepan. Add the remaining can of beans, stock, and sausage to the soup. Bring back to a boil, turn down to a simmer and add the chard and fresh herbs. Cook until the chard is wilted, about 1-2 minutes. Season to taste with salt and pepper.
Top with fresh grated Parmesan cheese, if desired (optional).
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