Heat oil in Dutch oven over medium-high heat until smoking. Add pancetta and cook, stirring occasionally, until pancetta begins to brown, 2 to 3 minutes. Stir in onion and chard stems and cook, stirring occasionally, until slightly softened, about 3 minutes. Add 1 tsp rosemary, garlic, and pepper flakes, and cook until fragrant, about 1 minute. Stir in beans and their liquid, 1.5 cups water and Parmesan rind, if using, and bring to boil. Reduce heat to medium-low and simmer for 10 minutes.