Prepare the Sauce Base:

Heat the olive oil in a large pan over medium heat. Add the chopped onion and sauté for 4-5 minutes, or until softened and translucent. Add the minced garlic and red pepper flakes (if using), and cook for another 30 seconds until fragrant.

Add the Flavorings:

Stir in the anchovy fillets (if using), capers, and olives. Cook for 2-3 minutes, stirring occasionally, until the anchovies dissolve and everything is well-mixed.

Simmer the Sauce:

Add the diced tomatoes and tomato paste to the pan, stirring to combine. Season with dried oregano, salt, and pepper. Let the mixture simmer over low heat for 10-15 minutes, stirring occasionally, to allow the flavors to meld and the sauce to thicken.

Add the Butter Beans:

Stir in the cooked butter beans and continue simmering for another 5-7 minutes, or until the beans are heated through and have absorbed some of the sauce. Taste and adjust seasoning with more salt or pepper if needed.

Serve:

Serve the Butter Bean Puttanesca in bowls, garnished with fresh basil or parsley and a sprinkle of grated Parmesan cheese (if desired).