Preheat the oven to 375°F. Grease a 9-inch pie dish with olive oil.
In a skillet over medium heat, add the olive oil and sauté the chopped onion until translucent, about 3-4 minutes.
Add the grated zucchini and cook for another 3-4 minutes until slightly softened. Remove from heat and let cool slightly.
In a large mixing bowl, whisk together the eggs, milk, oregano, garlic powder, salt, and black pepper.
Stir in the sautéed zucchini mixture, halved cherry tomatoes, and mozzarella cheese until well combined.
Pour the mixture into the prepared pie dish. If using, sprinkle the grated Parmesan cheese on top.
Bake in the preheated oven for 30-35 minutes, or until the quiche is set and lightly golden on top.
Allow to cool for 5-10 minutes before slicing.