In a covered sauce pan cook green beans in a small amount of boiling water until tender and crisp, about 15 minutes; drain and set aside.
For the white sauce, in a medium saucepan over medium heat melt butter. Stir in flour, dry salad dressing mix and white pepper until combined. Stir in milk. Cook and stir over medium heat until thickened and bubbly; remove from heat.
Spray a medium nonstick skillet with nonstick spray and warm over medium heat. Add onion and garlic to hot skillet; cook until tender. Remove half of the onion mixture and set aside. Add mushrooms to skillet and cook until tender.
In a 1 1/2-quart casserole, combine mushroom mixture, green beans and white sauce.
In a small bowl stir together reserved onion mixture and bread crumbs. Sprinkle over casserole and bake for 25 - 30 minutes or until heated through.