Heat the oven to 425°F. Keep thawed raw chicken separate from ready-to-eat food.
Arrange chicken, onion, carrots, celery and garlic on a rimmed pan. Roast 25 minutes, turn, then roast 15 to 20 minutes more until browned but not blackened.
Transfer solids to a 6-quart slow cooker. Deglaze the pan with 1 cup measured water and add the brown liquid.
Add bay leaves, peppercorns, vinegar and remaining water, staying below the maximum fill line and barely covering the bones.
Cover and cook on high for 1 hour without lifting the lid.
Switch to low and cook, covered, for 12 to 18 hours.
Remove large solids and strain gently without pressing. Season lightly or reduce on the stovetop if desired.
Divide into shallow containers and refrigerate promptly within 2 hours.