Preheat your oven to 400°F (200°C). If using raw chicken bones, spread them on a baking sheet and roast for 25-30 minutes, until nicely browned. This step is crucial for developing a rich, deep flavor and color. If using bones from a roasted chicken, you can skip this step.

Transfer the roasted bones (if applicable) to a large stockpot. Add the quartered onion, chopped carrots, chopped celery, parsley sprigs, thyme sprigs, bay leaf, and black peppercorns.

Pour in enough cold water to cover all the ingredients by about 1-2 inches. Starting with cold water helps to gradually draw out impurities and flavors.

Bring the pot to a boil over high heat. Once boiling, immediately reduce the heat to low, so the liquid is just gently simmering. Skim off any foam or impurities that rise to the surface during the first hour of simmering.

Cover the pot partially, leaving a small gap for steam to escape, and let it simmer gently for at least 4 hours, or up to 8 hours for a more concentrated flavor. Avoid a rolling boil, as this can make the broth cloudy.

Once simmering is complete, carefully strain the broth through a fine-mesh sieve set over a large bowl or another pot. Discard the solids. For an even clearer broth, you can line the sieve with cheesecloth.

Allow the broth to cool slightly, then you can use it immediately, or let it cool completely before storing. Once chilled, you can easily skim off any solidified fat from the surface if desired.