1) Cut onions, carrots, and rutabaga into quarters or sixths and combine in a large roasting pan with beef bones. Drizzle all the veggies and bones with some olive oil.
2) Roast the dish at 425 degrees for at least an hour, but 90 minutes would be okay depending on how dark you want your final stock.
3) Once roasted, add bones and veggies to a stock pot and top with celery, garlic, parsley, bay leaves, and peppercorns.
4) Add about a cup of water to the hot roasting pan and use the water to scrape up any bits stuck to the pan. Add those to the stock pot.
5) Add about 12 cups of cold water to the stock pot. The water should cover the bones and veggies.
6) Bring stock to a simmer on the stove and then let it simmer over low heat for 4-6 hours. The pot should never boil quickly. Make sure it is just a nice slow simmer.
7) After a few hours, a foam might start forming on the top of the stock. Scrape this off with a large spoon and discard it.
8) Once the stock has simmered as long as you want it to, strain stock through a sieve or cheesecloth. Discard the veggies and bones.
9) Let stock cool to room temperature and then store in the fridge overnight.
10) The next day, skim the fat off the top of the stock.
11) Store the stock in the fridge for 10 days or freeze for longer storage.