Start by removing the breasts from the whole chickens, keeping the legs, thighs, and carcasses.

Rinse the carcasses under cold water to eliminate any excess liquid.

Prepare the mirepoix by chopping the onion, celery, and carrot into large chunks, using a ratio of 50% onion, 25% celery, and 25% carrot.

Split the mirepoix in half; take one half and place it in a pan with a whole chicken carcass, then drizzle with olive oil and mix to coat.

Roast the pan in a 450°F (230°C) oven until the mixture is caramelized but not charred.

Once roasted, transfer the carcass and vegetables into a large pot and add 3–4 quarts of cold water.

Heat the pot over medium-low heat, allowing a light roast color to develop.

In another pot, combine the second chicken carcass with the remaining mirepoix and 3–4 quarts of cold water.

Simmer both pots, skimming off any foam that forms with a perforated spoon.

For the lighter (white) stock, simmer for 30–45 minutes before adding the mirepoix; for the brown stock, include the mirepoix from the beginning.

Add thyme and garlic cloves to both pots, and optionally include parsley stems or bay leaves.

Keep the heat at a gentle simmer with just a few bubbles, avoiding a rolling boil.

For deeper flavor, continue simmering the brown stock for up to four hours; the white stock can be finished after the shorter simmer.

Do not add salt at this stage; season later as needed.