Step 1: Heat the olive oil in a large, heavy-bottomed pot or Dutch oven over medium heat. Add the diced pancetta (if using) and cook until crispy, about 5-7 minutes. Remove the pancetta with a slotted spoon and set aside, leaving the rendered fat in the pot.
Step 2: To the pot, add the diced onion, carrots, and celery. Sauté for 8-10 minutes, or until the vegetables have softened and the onion is translucent.
Step 3: Stir in the minced garlic and red pepper flakes (if using) and cook for another 1-2 minutes until fragrant.
Step 4: Pour in the crushed San Marzano tomatoes, chicken or vegetable broth, bay leaves, dried oregano, and fresh rosemary sprig. Bring the mixture to a gentle simmer, then reduce the heat to low, cover, and let it cook for at least 20-30 minutes.
Step 5: Add the rinsed cannellini beans to the pot. Using a potato masher or the back of a spoon, gently mash about one-third of the beans against the side of the pot to thicken the soup.
Step 6: Stir in the ditalini pasta. Increase the heat to medium-low and cook, stirring occasionally, for 8-10 minutes, or according to package directions, until the pasta is al dente. If the soup becomes too thick, add a little more broth or water.
Step 7: Remove and discard the bay leaves and rosemary sprig. Stir in the chopped fresh parsley and season with salt and freshly ground black pepper to taste.
Step 8: Ladle the hot pasta fagioli into bowls. Garnish with the reserved crispy pancetta (if used), a generous sprinkle of grated Pecorino Romano or Parmesan cheese, and a drizzle of good quality extra virgin olive oil. Serve immediately with crusty bread.