Cook bacon in a large skillet until crisp. Transfer to a plate, leaving 1 tbsp fat in the pan.
Season chicken with salt and pepper. Brown skin-side down in bacon fat for 6 to 8 minutes until deep golden.
Spread onion in a 6-quart slow cooker. Top with chicken, skin-side up, and half the bacon.
Whisk cider, vinegar, mustard, and sage. Pour around chicken, not over it.
Cover; cook on low for 3 hours. Tuck cabbage around thighs.
Cover; cook 2 1/2 to 3 hours more until chicken reaches 175°F and cabbage is tender.
Skim sauce; scatter remaining crisp bacon over chicken and cabbage.