Use a spoon to scrape out the seeds and pulp in the center of the zucchini and pat dry with a paper towel.
Lay zucchini cut side up on a baking sheet.
Spread marinara sauce evenly between zucchini.
Top with mozzarella, mushrooms, pepperonis, green pepper, onion and olives.
Bake for 12-18 minutes. Cooking time will vary due to thickness of zucchini, but cook until cheese is melted and zucchini is tender.
Notes
Cooking time for baked stuffed zucchini can vary due to thickness. I used medium sized zucchini, which was about one and a half inches thick in diameter and it cooked perfectly at 400 degrees F. for 15 minutes.