Cook bacon in a large pot or Dutch oven over medium heat until crisp, about 8 minutes. Remove with a slotted spoon, leaving drippings.
Cook onion in drippings until softened and translucent, about 5 minutes, stirring occasionally.
Add greens in batches, letting each wilt slightly to make room for more.
Stir in broth, water, vinegar, and sugar; bring to a boil.
Reduce heat to low; add salt, pepper, and pepper flakes. Cover; simmer 60-75 minutes, stirring occasionally, until very tender.
Return bacon during last 10 minutes. Taste and adjust seasoning; serve with pot liquor.