In a large skillet over medium heat, cook the chopped bacon until crispy, about 8-10 minutes, stirring occasionally.
Remove the bacon with a slotted spoon and set aside, leaving the bacon fat in the skillet.
Add the sliced onion to the bacon fat and cook for 3-4 minutes until softened and translucent.
Add the garlic and cook for 30 seconds until fragrant.
Add the cabbage ribbons, salt, and black pepper to the skillet, tossing to coat with the bacon fat.
Cook for 12-15 minutes, stirring occasionally, until the cabbage is tender and beginning to caramelize with golden brown edges.
Stir in the crispy bacon and red pepper flakes if using, then serve immediately.