Chop the thick-cut bacon and place it in a large pot over medium heat. Cook until crispy and browned, about 7-10 minutes. Remove with a slotted spoon and set aside. Leave about 2 tablespoons of bacon fat in the pot.

Add chopped yellow onion to the bacon fat. Cook for 5 minutes or until translucent. Stir in the minced garlic and cook for another minute, until fragrant.

Cut the green cabbage into bite-sized pieces. Add it to the pot in batches, stirring to coat in the bacon-onion mixture. Once all cabbage is in the pot, season with salt, black pepper, and red pepper flakes if using.

Pour in chicken broth, reduce heat to low, and cover the pot. Simmer for 25-30 minutes, stirring occasionally, until the cabbage is tender.

Add the cooked bacon back into the pot along with the butter. Stir until melted and combined. Adjust seasoning to taste and serve warm.