Scrub the fennel and the potatoes and peel the onions. Cut them all into thick slices and saute gently in the oil in a large saucepan. Pour the hot stock or water over the vegetables, cover the pan and simmer for 20 minutes or so, until they are soft. Rub through a sieve or mouli-legumes. Heat through slowly stirring all the time, season and add the juice of one lemon. Mix the roasted fennel seeds with the grated lemon peel and sprinkle a little over each bowl.