Heat olive oil in a large pot or Dutch oven over medium heat. Add onion and cook until softened, about 5 minutes. Add garlic and red pepper flakes (if using) and cook for 1 minute more.
Add drained beans and chicken broth to the pot. Bring to a boil, then reduce heat and simmer for 30 minutes, or until beans are tender.
Add chopped collard greens to the pot and stir to combine. Cook for another 15 minutes, or until greens are tender.
Season with salt and pepper to taste. Serve hot.
Notes
Serve with crusty bread for dipping into the broth.