Cook the bacon in a large skillet until crisp, transfer it to a plate and leave 1 tbsp fat in the pan.
Season the chicken with the salt and pepper and brown it skin-side down in the bacon fat until deep golden, 6 to 8 minutes.
Spread the onion in a 6-quart slow cooker, set the chicken skin-side up on top and sprinkle in half of the bacon.
Whisk the cider, vinegar, mustard and sage and pour the mixture around, not over, the chicken.
Cover and cook on low for 3 hours, then tuck the cabbage around the thighs.
Cover again and cook until the chicken reaches 175°F and the cabbage is tender, 2 1/2 to 3 hours more.
Skim the sauce and scatter the remaining crisp bacon over the chicken and cabbage.