1. Preheat oven to 400°F. Rub a little olive oil over the bones, carrots and onions. Place in a large, shallow roasting pan. Roast in oven for about 45 minutes, turning the bones half-way through cooking until nicely browned.
2. Once browned, place bones and vegetables in a large (12 to 16 quart) stock pot. Place the roasting pan on the stove-top on low heat, pour 1/2 cup to a cup of hot water over the pan and use a spatula to scrape up all of the browned bits stuck to the bottom of the pan. Pour the browned bits and water into the stock pot.
3. Fill the stock pot with cold water – 1 to 2 inches over the top of the bones. Put the heat on high and bring the pot to a low simmer and then reduce the heat to low, with the stock at a bare simmer. Cover the pot loosely and let simmer for 3 – 6 hours. Do not stir the stock while cooking. Stirring will mix the fats in with the stock, clouding up the stock.
4. As the stock cooks, fat will be released from the bone marrow and rise to the top. From time to time, check in on the stock and use a large metal spoon to scoop away the fat and any scum that rises to the surface.
5. After cooking, use tongs or a slotted spoon to gently remove the bones and vegetables from the pot. Line another large pot (8-quart) with a fine mesh sieve, covered with a couple layers of cheesecloth if you have it. Pour the stock through the sieve to strain it. Let cool to room temperature then chill in the refrigerator.
Once the stock has chilled, any fat remaining will have risen to the top and solidified. The fat forms a protective layer against bacteria while the stock is in the refrigerator. If you plan to freeze the stock, however, remove and discard the fat. Leave an inch head room from the top of the stock to the top of the jar, so that as the stock freezes and expands, it will not break the container. (Makes about four quarts)