Preheat oven to 350. Coat a 2-quart square baking dish with nonstick cooking spray. Arrange thawed vegetables in dish. Set aside.
In a medium saucepan, whisk together evaporated milk, onion, flour, garlic, and black pepper. Cook and stir over medium heat until thickened and bubbly. Remove from heat. Add cheddar cheese and cream cheese, whisking until smooth.
Pour cheese mixture evenly over vegetables. Toss gently to coat vegetables with sauce. Sprinkle with bread crumbs. Lightly coat crumbs with additional cooking spray.
Bake 40-45 minutes or until mixture is bubbly and crumbs are lightly browned. Let stand 5 minutes before serving. If desired, sprinkle with parsley.