Preheat the oven to 375°F (190°C). Lightly grease a 9x13-inch baking dish.
Sauté the vegetables: Heat olive oil in a large skillet over medium heat. Add the onion, bell peppers, and zucchini. Cook for 4–5 minutes until slightly softened. Add the spinach and corn, and cook for another 2 minutes. Remove from heat.
Make the casserole base: In a large mixing bowl, combine the shredded chicken, sautéed vegetables, cream of chicken soup, sour cream (or yogurt), garlic powder, paprika, Italian seasoning, salt, and pepper. Add 1 cup of the shredded cheese. If using rice or pasta, stir it in now.
Transfer to baking dish: Pour the mixture into the prepared baking dish and spread it evenly.
Top with cheese: Sprinkle the remaining 1/2 cup of shredded cheese evenly on top.
Bake: Place in the oven and bake for 25–30 minutes, or until the top is golden and bubbly.
Garnish and serve: Remove from oven and let cool for 5 minutes. Garnish with fresh parsley or cilantro, if desired. Serve warm.