Preheat the Oven: Start by preheating your oven to 350°F (175°C). Grease a 9x9-inch baking dish or similar-sized casserole dish.
Prepare the Zucchini: Grate the zucchinis using a box grater or food processor. Place the grated zucchini in a clean kitchen towel or paper towels and squeeze out the excess moisture to prevent the casserole from becoming too watery.
Mix the Ingredients: In a large bowl, combine the cornbread mix, sour cream (or Greek yogurt), melted butter, egg, garlic powder, salt, and black pepper. Stir until the mixture is well combined.
Add the Zucchini: Gently fold the grated zucchini, shredded cheddar cheese (if using), and chopped green onions (if using) into the cornbread batter. Make sure the zucchini is evenly distributed throughout the mixture.
Transfer to Baking Dish: Pour the mixture into the prepared baking dish and spread it out evenly.
Bake: Place the casserole in the preheated oven and bake for 35-40 minutes, or until the top is golden brown and a toothpick inserted into the center comes out clean.
Serve: Let the casserole cool slightly before slicing and serving. It pairs well with grilled meats, roasted vegetables, or as a standalone dish.