Preheat oven to 375 degree F. In a covered saucepan, cook green beans in a small amount of boiling water for 10 to 15 minutes or until crisp-tender; drain and set aside.
Meanwhile, for white sauce:
In a medium saucepan, melt margarine. Stir in flour, dry dressing mix, and white pepper until combined. Stir in milk. Cook and stir over medium heat until thickened and bubbly; remove from heat.
Coat an unheated medium nonstick skillet with nonstick cooking spray. Preheat over medium heat. Add onion and garlic; cook for 2 to 3 minutes or until tender. Remove half of the onion mixture; set aside.
Add mushrooms to skillet and cook about 5 minutes or until tender. In a 1-1/2-quart casserole, combine mushroom mixture, beans, and white sauce.
In a small bowl, stir together reserved onion mixture and bread crumbs. Sprinkle bread crumb mixture over bean mixture in casserole. Bake, uncovered, for 25 to 30 minutes or until heated through. Makes 10 (1/2 cup) side-dish servings.