1. To make white sauce, melt butter in small saucepan over low heat. Stir in flour; cook 1 to 2 minutes, stirring constantly. Using wire whisk, stir in milk; bring to a boil. Cook, whisking constantly, 1 to 2 minutes or until thickened. Stir in dressing mix and white pepper; set aside.
2. Preheat oven to 350 degress F. Spray medium skillet with nonstick cooking spray; heat over medium-high heat. Add onion and garlic; cook and stir 2 to 3 minutes or until tender. Remove half of onion mixture; set aside.
3. Add mushrooms to onion mixture remaining in skillet and cook about 5 minutes or until mushrooms are tender. Combine mushroom mixture, green beans and white sauce in 1 1/2-quart casserole. Combine bread crumbs with reserved onion mixture; sprinkle over casserole. Bake, uncovered, until heated through, about 20 to 30 minutes.