You need a large heavy saucepan for this, with a well fitting lid. Cut the chicken into one-inch pieces. Scrub the clams or mussels and peel and finely chop the onions and garlic. Heat the oil in the saucepan and fry the onion and garlic for two minutes, then add the chicken and cook over a medium heat for 10 minutes with the lid on, turning occasionally to make sure it is thoroughly cooked. Add the shellfish, replace the lid and cook for five minutes, shaking hard every minute or so until the clam or mussel shells have opened. Discard any that won't open. Serve in a warm dish with parsley sprinkled on top.