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

Place chicken bones on the prepared baking sheet.

Roast in the preheated oven for 30 minutes.

Transfer bones to a large, heavy stockpot. Add onion and enough water to cover by 2 inches. Add vinegar, salt, and black pepper; bring to a boil over high heat. Reduce heat to medium-low; simmer, partially covered, leaving enough space for steam to escape, for 4 to 5 hours. Check occasionally, remove any froth or foam that develops on top of water with a large spoon.

Remove from the heat; cool slightly. Strain broth through a fine-mesh sieve into a clean pot. Cool completely at room temperature, then store in the refrigerator. Use within 5 days or freeze until ready to use.