Preheat oven to 450.
Place bones in a large roasting pan in a single layer and sprinkle onions and carrots around them.
Roast for 1 hour or until the bones have caramelized.
Transfer bones and vegetables to a large stock pock and set aside.
Put the roasting pan on the stove top over high heat. Add about 2-3 cups of water to the pan. Bring to a boil and deglaze the pan. Scrap any of the brown bits from the pan with a wooden spoon. Pour liquid and brown bits from the roasting pan into the stock pot.
Cover the bones in the stock pot with about 2 inches of water, and place the bouquet garni in the center.
Bring to a gentle simmer. Simmer gently, uncovered for 24 hours.
Add water as needed to keep the bones covered.
Remove from heat.
Discard the bones, and strain the liquid through a fine mesh strainer.
Let cool to room temperature and lift the fat off.
Place liquid into your storage containers, such as jars or ice cube trays.