01 - Preheat oven to 400°F. Lightly grease a 9x13-inch baking dish with cooking spray or butter.
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 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 cooked chicken, sautéed vegetables, sour cream, milk, dried oregano, dried basil, salt, and pepper. Stir until evenly incorporated.
05 - Spread the chicken and vegetable mixture evenly across 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, crispy crust.
07 - Bake in the preheated oven for 25 to 30 minutes, or until the casserole is bubbly and the top is golden brown.
08 - Remove from the oven and let the casserole rest for 5 minutes before slicing and serving.