Put cut-up chicken and onion into the water and cook slowly until tender, add salt. Remove chicken, cut the meat into small (1-inch) pieces and return to broth, together with corn, which has been cut from the cob, celery and seasoning. Continue to simmer. Make rivels by combining 1 cup flour, a pinch of salt, 1 egg and a little milk. Mix well with fork or fingers to form small crumbs. Drop these into the soup, also the chopped, hard-boiled eggs. Boil for 15 minutes longer.