Heat a large Dutch oven or other large pot over medium-high heat. Fry the bacon until it’s starting to brown, about 5 minutes.
Reduce the heat to medium; add the onion. Cook stirring regularly, until the onion softens, about 7-10 minutes. Add the garlic and cook until fragrant, about 45-60 seconds.
Add the tomatoes with their juices, broth, and red pepper. Bring to a boil, then reduce the heat to maintain a simmer. Cook, stirring occasionally, for 20 minutes.
Return the liquid to a boil. Add the chopped collard greens in batches, waiting until they wilt to add the next batch. Stir in the vinegar, brown sugar, and salt to taste.
Cook, stirring occasionally, until the greens are just tender, about 15-20 minutes. Taste and season with additional red pepper flakes, brown sugar, or salt, if desired.
Notes
Buy pre-washed and pre-chopped collard greens for a shortcut.
It's important to clean greens really well before using, since dirt, grit, sand, and soil can get trapped in them. To clean them, fill a sink or large bowl with cold water. Add the greens and swish them around so that any dirt falls to the bottom of the sink. Pull out the greens, rinse out the sink, and repeat the process until the greens are completely clean.
If using canned tomatoes, I like to coarsely chop them with kitchen shears as I add them to the pot.
Adjust the total cooking time to suit your preferences. I simmer these greens just until tender, which only takes about 15-20 minutes. If you like very broken-down, very tender greens (not at all crisp!), you may like to cook them for up to 2 hours. That’s the beauty of using a Dutch oven (rather than a slow cooker or Instant pot) — you have full control and can easily take them off of the heat when necessary.