Chop the cabbage into 1-inch pieces after removing the tough core. Slice the onion thinly and mince the garlic. Have all ingredients prepped before cooking.
Add the chopped bacon to a large, cold skillet or Dutch oven, then turn heat to medium. Cook for 5-7 minutes, stirring occasionally, until the bacon is crisp and the fat has rendered. Remove about half the bacon with a slotted spoon and set aside for garnish.
Add the sliced onions to the bacon fat and cook on medium heat for 5-7 minutes until they become translucent and begin to caramelize. Add the minced garlic during the last minute of cooking.
Increase heat to medium-high and add the chopped cabbage to the skillet. Toss to coat with the bacon fat and onion mixture. If your pan seems dry, add the optional butter. Season with salt, pepper, and red pepper flakes.
Cover and cook for 10 minutes, stirring occasionally. Once the cabbage begins to soften, remove the lid and continue cooking for another 5-7 minutes until the cabbage reaches your desired tenderness.
Stir in apple cider vinegar and optional brown sugar. Cook for 1-2 minutes more to integrate flavors.
Transfer to a serving dish, top with reserved crispy bacon pieces and fresh parsley. Serve immediately.