Cook bacon in a large skillet or Dutch oven over medium heat for 10-12 minutes, or until almost crisp. Add onion, and sauté until translucent, about 7-8 minutes.
Stir in garlic and rosemary (or thyme), sauté 1 more minute. Add red pepper flakes, broth, greens, vinegar, sugar, ¼ teaspoon salt, and ¼ teaspoon pepper. Use a wooden spoon to stir, scraping up any browned bits from the bottom of the pot.
Stir in the beans. Bring the liquid to a boil, then turn the heat to low and simmer, mashing some of the beans with a wooden spoon, until the liquid has reduced and thickened, 6 to 8 minutes.
At this point, the greens should be wilted and slightly tender. If you prefer the greens even more tender, you can cover the pot and let the mixture gently simmer until the greens reach the desired level of tenderness.
Taste and season with additional salt and pepper, if necessary. Serve with cornbread.