Preheat your oven to 375 degrees F. In a pan over medium heat, sauté your onion, garlic, mushrooms, and spinach in a bit of olive oil over medium high heat.
Prep your zucchini: Wash zucchini, trim ends, then slice in half lengthways. Then, using your knife, a small spoon, or a melon baller, scoop out the seeds.
Place the zucchini halves onto a prepared baking sheet (flesh side up) and fill the center of each zucchini half with marinara sauce, your cooked mushroom mix, and sliced olives.