Place the zucchini and onion in a steamer basket. Place in a saucepan over 1 inch of water; bring to a boil. Cover and steam for 4-6 minutes or until crisp-tender; drain.
Combine eggs, bouillon, Italian seasoning, mustard and pepper; set aside.
Layer half of zucchini mixture in a 1 quart baking dish coated with cooking spray. Sprinkle with 1/2 cup Swiss cheese. Repeat layers. Pour egg mixture over top.
Bake, uncovered, at 350 degrees for 25 to 30 minutes or until a knife inserted near the center comes out clean.