In the bowl of your crockpot, combine the chicken stock, butter, olive oil, onions, carrots, parsnips, celery, leeks, peppercorns, bay leaf, thyme, rosemary, and a pinch each of salt and pepper. Add the chicken. Cover and cook on low for 5-6 hours or high for 4-5 hours. About 45 minutes before serving, remove the chicken from the soup. Remove the herbs and discard. Stir in the lemon juice and zest, plus the farro. Shred the chicken and stir it back into the soup and cook until the farro is tender.Divide the soup among bowls and top with herbs. Serve with a hunk of bread.