01 - Preheat oven to 400°F. Lightly grease a 9x13-inch baking dish with olive oil or cooking spray.
02 - Heat olive oil in a large skillet over medium heat. Add the chopped onion and sauté for 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. 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 everything is evenly coated.
05 - Spread the chicken and vegetable mixture evenly across the prepared baking dish. Layer the shredded mozzarella and grated Parmesan cheese over the top.
06 - Sprinkle breadcrumbs evenly over the cheese layer to create a golden, crispy crust.
07 - Bake uncovered for 25 to 30 minutes, or until the cheese is bubbly and the topping is golden brown.
08 - Remove from the oven and let rest for 5 minutes before serving to allow the layers to set.