Slice zucchini; set it aside.
Slice mushrooms; set them aside.
Chop onions; combine with zucchini.
Heat butter or oil in skillet; saute the zucchini and onions until just tender.
Add the mushrooms; cook for about 30 seconds more.
Add salt and pepper to season.
Add beaten eggs to mixture in skillet.
Cook on medium heat until bottom is browned.
Carefully turn omelet, cover, and cook until browned on other side.