Return carcass and bones to cooking water. Add onion trimmings, celery, carrot, a bay leaf, and a splash of vinegar. Add enough water to cover. Simmer gently for 3-4 hours on stove or 24-48 hours in the crock pot on low. Strain solids, reserving stock. Add salt to taste. Discard or compost solids. Use this delicious golden stock in any recipe calling for chicken broth. Store in fridge for a few days. Or, reduce (by boiling down) and freeze in ice cube trays for future use.