01 - Preheat oven to 400°F. Grease a 9x13-inch baking dish with cooking spray or a small amount 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. Remove from the skillet and set aside on a plate.
03 - In the same skillet, add diced onion and minced garlic. Sauté for 2–3 minutes until fragrant and softened, scraping up any browned bits from the chicken.
04 - Add zucchini slices to the skillet and cook for another 4 minutes until slightly tender but still holding their shape. Remove from heat.
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 everything is evenly coated.
06 - Spread the mixture evenly in the prepared baking dish. Top with remaining mozzarella and Parmesan cheeses. Sprinkle breadcrumbs evenly over the top.
07 - Bake uncovered for 25–30 minutes, or until the top is golden brown, the cheese is bubbly, and the filling is hot throughout.
08 - Let the casserole rest for 5 minutes before serving to allow the filling to set. Serve warm.