3 5 pound whole roasting chickens 3 large yellow onions 6 carrots, unpeeled, halved 4 celery stalks with leaves, cut in thirds, 4 parsnips, unpeeled, cut in half (optional) 20 sprigs fresh flat-leaf parsley 15 fresh thyme 20 sprigs fresh dill 1 head garlic, unpeeled, cut in half crosswise 2 tablespoon kosher salt 2 teaspoon whole black peppercorns 7 quart water