Soften the onions and carrots in the olive oil in a heavy, covered pan. Wash and scrape the celery sticks, split the main part of each one vertically and cut it into 2.5 cm (1 in) lengths. Stir the celery into the pan and leave it to steam-cook for 20-30 minutes over a low heat to bring out the flavour. Add the washed and diced potatoes, cover with the water, season with a little salt and a few black peppercorns, bring to the boil and simmer for 20 minutes. Add the chopped parsley and liquidize.