01 - Preheat oven to 400°F. Grease a 9x13-inch baking dish with cooking spray or a thin layer of oil.
02 - Heat olive oil in a large skillet over medium heat. Add diced chicken and cook for 5–6 minutes until just cooked through and no longer pink in the center. Remove from skillet and set aside.
03 - In the same skillet, add diced onion and minced garlic. Sauté for 2–3 minutes until fragrant and softened, stirring occasionally.
04 - Add zucchini slices to the skillet and cook for another 4 minutes until slightly tender but still firm.
05 - In a large bowl, combine cooked chicken, sautéed vegetables, sour cream, milk, half of the mozzarella, half of the Parmesan, Italian herbs, paprika, salt, and pepper. Mix until well combined and evenly coated.
06 - Spread the mixture evenly in the prepared baking dish, pressing lightly to create an even layer.
07 - Top with remaining mozzarella and Parmesan cheeses. Sprinkle breadcrumbs evenly over the entire surface.
08 - Bake uncovered for 25–30 minutes, or until the top is golden brown, the cheese is bubbling, and the filling is hot throughout.
09 - Let the casserole rest for 5 minutes before serving to allow the filling to set slightly.