Add bacon, in one layer, to a large skillet over medium heat. Cook, stirring occasionally, until the fat renders and the bacon is crispy, 6 to 8 minutes.
Take the pan off of the heat, and then use a slotted spoon to transfer the bacon to a bowl and save for later.
Remove all the bacon fat from the pan except for one tablespoon. Leave more or less, depending on your preference. One tablespoon makes the cabbage taste great but won’t make the cabbage greasy.
Place the pan back onto medium-high heat, then add the cabbage, garlic, red pepper flakes, and salt. It might seem like too much cabbage for the pan, but as it cooks, the cabbage will wilt.
Cook, occasionally stirring, until the cabbage is tender and some of it begins to turn light brown, 10 to 15 minutes.
Stir in the mustard, apple cider vinegar, and the reserved cooked bacon.
Taste and adjust with more salt, pepper, and vinegar as needed.
