Heat a medium Dutch oven over medium-high heat. Add 2 tablespoons olive oil to the pan along with the onion, celery, carrot and garlic. Season with the salt and cook, stirring often for about 4 minutes, or until fragrant and soft. Add the red pepper flakes, if using, and cannellini beans and stir to combine.
Add the broth and bring to a simmer. Reduce the heat to medium to maintain a gentle simmer and cook for 5 minutes. Add the escarole and stir to wilt. Cook an additional 5 minutes. Serve topped with a drizzle of the remaining olive oil, fennel pollen and Parmigiano-Reggiano, if using.