Cook onion, garlic, and chopped bacon in a large Dutch oven over medium-high heat 6 minutes or until tender. Add 4 cups water and next 5 ingredients (through salt); bring to a boil. Gradually add greens. Cover, reduce heat, and simmer 45 minutes, stirring occasionally. Uncover; cook 1 hour or until very tender. Serve with pepper vinegar, if desired.