Step 1

Heat oil in a large heavy pot over medium. Add bacon, if using, and cook, stirring occasionally, until some of the fat begins to collect in the pot and bacon starts to look shiny, about 5 minutes. Add onion and cook, stirring occasionally, until golden and softened, 5–7 minutes. Add garlic and red pepper flakes and cook, stirring, until fragrant, about 30 seconds.

Step 2

Working in batches, add collard greens to pot, letting greens wilt slightly after each handful before adding more. Add broth, bring to a simmer, and cook, stirring often, until greens wilt and lose about half of their original volume, about 4 minutes. Reduce heat to medium-low, partially cover pot, and simmer gently, stirring occasionally, until greens are tender and liquid is almost completely evaporated, 25–30 minutes. Stir in vinegar and sugar and cook another minute to allow sugar to dissolve; season with salt and pepper.