In a large pot, cook chopped bacon over medium heat until crispy, about 10 minutes. Remove bacon and set aside; keep bacon grease in the pot.
Add chopped onion and cook until soft, about 4 minutes. Stir in sliced garlic and red pepper flakes, and cook until fragrant, about 1 minute.
Add green beans and chicken broth with a pinch of salt. Bring to a boil, then reduce heat, cover, and simmer for 45-60 minutes until very tender. Add more broth if needed during cooking.
Stir in butter and reserved bacon. Season with salt and pepper to taste. Serve warm.
Notes
These beans get even better the longer they simmer—just keep an eye on the liquid level. They’re a great make-ahead side!