Preheat the oven to 375°F (190°C).

In a large mixing bowl, combine diced zucchini, bell peppers, chopped onions, and grape tomatoes. Sprinkle with Italian seasoning, garlic powder, salt, and pepper. Drizzle with olive oil and mix well.

Fold half of the mozzarella cheese into the vegetable mixture and transfer it to a greased baking dish.

Bake in the preheated oven for about 20 minutes until the zucchini is tender and slightly golden.

Sprinkle the remaining mozzarella cheese on top and bake for an additional 10 minutes until melted and bubbly.

Let cool for a few minutes, garnish with parsley and chili flakes if desired, and serve.