In a large stockpot over medium-high heat add the bones, carrots, celery, white onions, smashed garlic cloves, turnip, parsnips, bay leaves, thyme leaves, and black peppercorns. Pour in just enough cold water to cover the ingredients. Bring to a boil, reduce heat to low and allow to simmer partially covered for 2 hours. As it cooks, skim the foam off of the top to remove any impurities. Add a little more water if necessary to keep the vegetables and chicken bones covered while simmering. Being careful not to add too much, as it will dilute the stock.