Preheat the oven to 450 degrees F (230 degrees C). Line a baking sheet with aluminum foil.

Place chicken bones onto the prepared baking sheet.

Place baking sheet in the preheated oven and roast bones for 30 minutes.

Gently transfer bones from the baking sheet into a large and heavy stockpot. Add onion and enough water to cover the bones by 2 inches. Add vinegar, salt, and pepper and bring to a boil over high heat. Reduce heat to medium-low and simmer, partially covered, leaving enough space for steam to escape, for 4 to 5 hours. Check occasionally for froth or foam that develops on top of the water, removing it with a large spoon.

Remove from the heat and let cool. Strain broth through a fine mesh strainer into a clean pot. Let cool completely. Store in the refrigerator and use within 5 days or freeze until ready to use.