01 - Preheat oven to 400°F. Lightly grease a 9x13-inch baking dish with olive oil or nonstick spray.
02 - Heat olive oil in a large skillet over medium heat. Add the finely chopped onion and sauté for about 2 minutes until softened and translucent.
03 - Add the minced garlic, sliced zucchini, and diced red bell pepper to the skillet. Sauté for 5 to 6 minutes until the vegetables are just tender but still hold their shape. Remove from heat and set aside.
04 - In a large mixing bowl, combine the shredded chicken, sautéed vegetables, sour cream, milk, dried oregano, dried basil, salt, and pepper. Stir thoroughly until all ingredients are evenly distributed.
05 - Spread the chicken and vegetable mixture evenly across the bottom of the prepared baking dish. Top with an even layer of shredded mozzarella and grated Parmesan cheese.
06 - Sprinkle breadcrumbs uniformly over the cheese layer to create a golden, crunchy crust during baking.
07 - Bake on the center rack for 25 to 30 minutes, or until the cheese is melted and bubbly and the topping is golden brown.
08 - Remove from the oven and let the casserole rest for 5 minutes to allow the filling to set before slicing and serving.