Top up with cold water, add half the onion, bring to the boil and season with salt. Lower the temperature and let the chickpeas simmer gently for about 90 minutes.
Drain and puree the peas (not half the onion!) With about half of the oil, adding salt and pepper if necessary.
Put the dish in a deep plate and pour the remaining oil over it. Serve sprinkled with a few onion rings and parsley.