Add the chicken to a large stock pan and fill with water until chicken is covered by about 1 inch (usually about 1 gallon of water).: The kitchen will smell faintly of raw chicken and metal as the pieces nestle into the pot, the water just covering them so there is room for gentle movement as it heats. You want enough water to extract flavor without diluting it, which is why about 1 gallon is a practical target for 4 lbs of pieces. A common mistake is overfilling which leads to weaker extraction, so check that pieces are submerged but not swimming in excess water. Use a wide pan so heat distributes evenly, and arrange the pieces so air pockets are minimized for steady, even simmering.
Bring water to a simmer over medium-high heat and cook for 30 minutes, skimming any impurities that float to the top.: As the pot warms, watch for tiny bubbles appearing along the edge, the first sign you are nearing a simmer. You will see pale scum rise to the surface, which you should gently skim with a spoon for clearer broth. The aroma shifts from neutral to savory and faintly sweet as proteins begin to denature. Avoid a rolling boil, which agitates the contents and clouds the liquid; a tame, steady simmer yields clarity and a cleaner flavor profile. An easy trap is leaving the pot unattended until it boils vigorously, which pulls fat and broken proteins into the broth and increases cloudiness.
Make the bouquet garni by placing the herbs on a square piece of cheesecloth and fold up the edges to form a pouch. Tie together with string, allowing enough string to easily pull the bouquet garni from the pan when needed.: Handling the herbs, you will smell sharp, green notes from the thyme and rosemary, and a fresh lift from parsley. Wrapping them in cheesecloth gives you the benefit of infused flavor while keeping tiny leaves and stems contained. Leaving a tail of string ensures you can retrieve the bundle without fishing. If you skip the pouch and just toss herbs in, you risk small bits floating around and clouding the broth, so the pouch technique protects both texture and appearance.
Carefully add the mirepoix, bay leaves, and bouquet garni into the pot. Simmer for an additional 2 hours, skimming impurities that rise to the surface.: When the onions, carrots, and celery join the pot, the scent deepens into a savory aromatic chorus, with sweet, vegetal edges. Over the next two hours the broth will reduce slightly and concentrate, becoming more golden and fragrant. Keep the simmer low, and periodically lift off any foam with a slotted spoon to maintain clarity. If you let the heat creep up, the broth can become cloudy and taste harsher, so adjust the burner as needed. Patience here builds complexity, so resist the urge to crank the heat to speed things up.
Use a pair of tongs to remove the large pieces of chicken from the broth into a large bowl (the chicken can be pulled from the bones and saved for another use). Carefully strain the broth through a colander into a separate pot. Season with salt.: Removing the solids reveals a glossy, amber liquid with a clean aroma. Pull the meat from the bones while warm if you plan to use it, noting it may be a bit stringy but very usable. Straining removes vegetables and any stray solids, leaving you with a clear stock. At this stage, tasting and seasoning matters because concentration during simmering changes perceived saltiness. Add a modest amount of Kosher salt , taste, then adjust after chilling. A common misstep is over salting before reduction, so err on the side of less, then finish later.
Place in the refrigerator for at least 24 hours. Remove from the fridge and use a spoon to skim off the congealed fat that has formed on the surface. The broth is now ready to use.: Chilling turns the surface fat into a pale solid layer you can skim away for a leaner broth, or you can reserve the fat for cooking. The broth beneath will be gelled slightly if bones released enough collagen, which is a wonderful textural sign of success. When reheating, the aroma will bloom again, and you will notice a rounded mouthfeel. A common oversight is not chilling long enough before skimming, which makes fat removal harder and leaves a greasier mouthfeel in the final dish.
