01 - Preheat oven to 400°F. Lightly grease a 9x13-inch baking dish.
02 - Heat olive oil in a large skillet over medium heat. Add onion and sauté for 2 minutes until softened and translucent.
03 - Add garlic, zucchini, and red bell pepper to the skillet. Sauté for 5 to 6 minutes until vegetables are just tender. Remove from heat and set aside.
04 - In a large bowl, combine the cooked chicken, sautéed vegetables, sour cream, milk, oregano, basil, salt, and pepper. Stir until evenly mixed.
05 - Spread the chicken and vegetable mixture evenly in the prepared baking dish. Top with an even layer of mozzarella and Parmesan cheese.
06 - Sprinkle breadcrumbs evenly over the cheese layer for a golden, crunchy finish.
07 - Bake for 25 to 30 minutes, or until the casserole is bubbly and the top is golden brown.
08 - Remove from the oven and let rest for 5 minutes before serving to allow the filling to set.