In a large frying pan, heat olive oil over medium heat. Cook the zucchini and onion until the zucchini is tender and the onions are translucent. Drain in a colander and allow veggies to cool for 5–10 minutes.
Meanwhile, whisk the eggs, mustard, shredded mozzarella, and seasonings until combined in a large mixing bowl. When the zucchini has cooled slightly, stir in the zucchini/onion mixture.
Grease a 9x13 casserole dish. Pour in zucchini/egg mixture. Smooth over the zucchini so it lays flat.
Bake for 30–35 minutes or until eggs start to brown. Cover with foil and bake for an additional 10 minutes or until a knife is inserted into the center and comes out clean.
Let the casserole stand for 10 minutes before serving.
Storage: Store cheesy zucchini egg bake covered in the refrigerator (baked) for three to four days or in the freezer (baked) for two to three months. Reheat in the microwave in 30-second intervals.