PREHEAT OVEN TO 350F. Place pancetta in an oven-proof pot over medium heat and cook, stirring occasionally, for 2 minutes. Add the onion, carrot, celery and garlic and continue to cook, stirring occasionally, another 7 minutes, or until vegetables soften. Add the beans, broth and thyme. Cover tightly, place in oven. Bake for 1 1/2 hours or until beans are soft. Meanwhile, remove center stem from the chard leaves. Slice into crosswise slices 1/4-inch thick. Roughly chop the chard leaves. Add the chard to the bean mixture and replace in the oven. If the soup is looking too thick, add some water. Cook another 20 minutes. Remove the pot from the oven and add salt and pepper as needed. Serve the soup piping hot and offer grated Parmesan cheese on the side.