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 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. Sprinkle the shredded mozzarella and grated Parmesan cheese in an even layer over the top.
06 - Scatter the breadcrumbs evenly over the cheese layer to create a golden, crispy finish during baking.
07 - Bake uncovered for 25 to 30 minutes, or until the casserole is bubbly around the edges and the top is golden brown.
08 - Remove from the oven and let the casserole rest for 5 minutes before slicing and serving. This allows the filling to set for cleaner portions.