Place bacon pieces in a large cold 12-inch cast-iron skillet or heavy-bottomed pan. Turn heat to medium and cook, stirring occasionally, until the bacon is crispy and the fat has fully rendered, about 8 to 10 minutes.

Use a slotted spoon to transfer the crispy bacon to a paper-towel-lined plate. Leave all the rendered bacon fat in the skillet — you should have about 2 tablespoons. If you have significantly more, pour off the excess, leaving just enough to coat the bottom of the pan.

Increase heat to medium-high. Add the sliced yellow onion to the hot bacon fat and cook, stirring occasionally, until the onion is softened and beginning to turn golden at the edges, about 4 to 5 minutes.

Add the minced garlic and smoked paprika (if using) and cook for 30 seconds, stirring constantly, until fragrant. Do not let the garlic brown.

Add the chopped cabbage to the skillet in large handfuls, pressing it down as it wilts to make room for the rest. Season with salt, black pepper, and red pepper flakes if using. Stir to coat everything in the fat.

Cook the cabbage over medium-high heat, stirring every 2 to 3 minutes and allowing it to sit undisturbed between stirs so it develops golden-brown caramelized edges. Continue cooking until the cabbage is tender, wilted, and deeply golden in spots, about 12 to 15 minutes total.

Remove the skillet from heat. Add the apple cider vinegar and stir to combine — it will sizzle and deglaze any browned bits from the bottom of the pan. Taste and adjust salt and pepper as needed.

Return the crispy bacon pieces to the skillet, stir once more to combine, and serve immediately directly from the skillet.