In a large skillet or pot, heat the olive oil over medium heat.
Add the Italian sausage and cook, breaking it up with a spoon, until browned and cooked through (about 5-7 minutes).
Stir in the chopped onion and cook until softened. Add the minced garlic and cook for another minute.
Gradually add the chopped escarole, stirring until it wilts down.
Stir in the diced tomatoes, cannellini beans, and chicken or vegetable broth.
Add oregano, salt, pepper, and red pepper flakes. Bring to a simmer and cook for about 10-15 minutes to meld the flavors.
Before serving, stir in the lemon juice for brightness.
Ladle into bowls and top with grated Parmesan cheese if desired.