In a large pot or Dutch oven, cook the chopped bacon over medium heat until crispy, about 8 minutes. Remove bacon with a slotted spoon and set aside, leaving the drippings in the pot.
Add the diced onion to the bacon drippings and cook until softened and translucent, about 5 minutes, stirring occasionally.
Add the chopped collard greens to the pot in batches, allowing each batch to wilt slightly before adding more. This may take several additions to fit all the greens.
Pour in the chicken broth, water, apple cider vinegar, and sugar. Stir to combine and bring to a boil.
Reduce heat to low, add the salt, black pepper, and red pepper flakes. Cover and simmer for 60-75 minutes, stirring occasionally, until greens are very tender.
Stir the reserved crispy bacon back into the greens during the last 10 minutes of cooking. Taste and adjust seasoning as needed before serving with the pot liquor.