Peel the asparagus and slice the fennel after removing its stalks. Place the asparagus in a large pot of boiling salted water.
Bring to a boil and cook for about 15 minutes (depending on the size of the asparagus). Steam the fennel simultaneously. When the vegetables are cooked, remove the cooking water and set aside.
Whip the egg whites with a mixer and a pinch of salt.
Heat a frying pan with the fat, place the omelette in it and brown.