Place bacon in a large frying pan and cook over medium-high heat, stirring occasionally, until browned and fat is rendered, about 3 minutes. Add onion and cook until softened, about 4 minutes. Add greens and turn to coat well. Add broth – it should cover greens and let cook until broth is reduced and greens are very tender, about 15 minutes. Season with salt and freshly ground black pepper and serve.