In a large pot, combine the chicken, quartered onion, parsnip chunks, carrot chunks, celery chunks, garlic, thyme, the 3 whole parsley sprigs, the 6 whole dill sprigs, the bay leaves, and the peppercorns. Add cold water to cover and come up just below the top of the pot, about 5 quarts. Bring to a boil and then reduce the heat to simmer at a low bubble, uncovered, until the chicken is very tender, about 1 1/2 hours (or longer if you have the time — up to 6 hours, topping off with more water if the stock dips below the chicken and veggies), skimming off any scum (there won’t be much) and, if desired, some fat.