Preheat the oven to 425 degrees (220 degrees C).

Arrange the chicken parts, carrots, celery, garlic, and onion in a roasting pan large enough to hold them in one layer; drizzle all those components with the oil and season well with kosher salt. Roast for 30 to 45 minutes or until the skin on the chicken is golden brown and the vegetables are browning.

Combine the roasted chicken and vegetables with the black peppercorns, rosemary, thyme, bay leaves, and sage in a large stockpot or Dutch oven. Add the cold water to the pot and bring to a lively simmer over medium heat. Then reduce the heat to medium-low, letting the stock simmer gently; partially cover and cook for anywhere from 3 hours to 8 hours.

As foam and fat rise to the top, skim it off using a spoon. Repeat every so often.

Taste, and add salt and ground black pepper, as needed. If using, add the juice of a lemon.

Use a slotted spoon to remove the chicken, allowing the stock to drain back into the pot. You can reserve the chicken meat for other dishes; it will keep in a covered container in the refrigerator for 3 days.

Strain the broth in a mesh colander and cheesecloth, discarding the remaining vegetable solids, bones, and chicken skin. Cool the broth in a bowl in the fridge covered in plastic wrap. Let chill for 8 hours to overnight, then skim the excess fat that has chilled and risen to the top with a spoon and discard it.

Freeze stock flat in freezer-safe zip-top bags, it'll last for six months in the freezer. Otherwise keep it in a covered container for 4-5 days in the fridge.