Fill a large basin with water and add escarole; drain basin, refill with water, and return escarole to basin. Repeat this process at least 4 times, or until basin is free of sand. Drain, and chop escarole into bite-size pieces.

Bring a large pot of water to a boil. Add escarole and cook 20 minutes. Drain.

Heat oil in a Dutch oven over medium heat. Add garlic and cook until golden brown, 6 to 8 minutes. Add beans in their liquid, oregano, red-pepper flakes, chicken broth, and escarole. Raise heat and bring soup to a simmer; cover and cook 15 minutes.

Place biscotti in individual serving bowls. Ladle soup into bowls and garnish with Parmesan cheese.