Follow these simple steps to create a Chicken Zucchini Casserole that will impress your family and friends. This recipe is designed to be straightforward and easy to follow, even for beginner cooks.

1. Preheat the Oven and Prepare Baking Dish: Start by preheating your oven to 375°F (190°C). This ensures the oven is at the correct temperature when you place the casserole in, allowing for even cooking. Grease a 9×13 inch baking dish with cooking spray or a little olive oil. This prevents the casserole from sticking and makes serving easier.
2. Sauté Onion and Garlic: Heat olive oil in a large skillet over medium heat. Add the diced onion and cook until softened and translucent, about 5-7 minutes. Stir occasionally to prevent burning. Add the minced garlic and cook for another minute until fragrant. Be careful not to burn the garlic as it can become bitter. Sautéing the onion and garlic first releases their flavors and creates a flavorful base for the casserole.
3. Add Chicken and Cook: Add the cubed chicken breast to the skillet with the onion and garlic. Season with salt, pepper, and Italian seasoning. Cook until the chicken is cooked through and no longer pink, about 8-10 minutes. Ensure the chicken is cooked evenly for food safety. Breaking the chicken into smaller pieces as it cooks helps it cook faster and distributes it evenly throughout the casserole.
4. Prepare Zucchini: While the chicken is cooking, prepare the zucchini. Wash the zucchini thoroughly and trim off the ends. You can slice the zucchini into rounds or half-moons, about ¼ inch thick. No need to peel the zucchini as the skin is tender and nutritious. If you prefer, you can lightly sauté the zucchini slices in a separate pan for a few minutes to remove some excess moisture, but this step is optional.
5. Combine Ingredients in a Bowl: In a large bowl, combine the cooked chicken mixture, sliced zucchini, cream of chicken soup, sour cream, and milk. Stir everything together until well combined and the zucchini and chicken are evenly coated in the creamy sauce. Ensure all ingredients are well mixed to create a cohesive casserole.
6. Layer Cheese and Assemble Casserole: Pour half of the cheddar cheese and half of the mozzarella cheese into the chicken and zucchini mixture and stir to combine. This incorporates cheese throughout the casserole for a richer, cheesier flavor. Pour the mixture into the prepared baking dish, spreading it evenly. Top the casserole with the remaining cheddar and mozzarella cheese, creating a cheesy crust on top.
7. Bake the Casserole: Bake in the preheated oven for 25-30 minutes, or until the casserole is bubbly, heated through, and the cheese is melted and golden brown. The baking time may vary slightly depending on your oven. The casserole is ready when it’s heated through and the cheese is nicely melted and browned.
8. Broil for Extra Browning (Optional): For a more golden brown and bubbly cheese topping, you can broil the casserole for the last 1-2 minutes of baking. Keep a close eye on it to prevent burning. Broiling adds a beautiful finish to the casserole.
9. Rest Before Serving: Remove the casserole from the oven and let it rest for 5-10 minutes before serving. This allows the casserole to set slightly and makes it easier to slice and serve. Resting also helps to prevent burning your mouth on piping hot cheese!
10. Garnish and Serve: Garnish with optional toppings like breadcrumbs or fresh parsley, if desired. Serve hot and enjoy this comforting and flavorful Chicken Zucchini Casserole!