PREHEAT oven to 425° F. Spray 13 x 9-inch or 3-quart casserole dish with nonstick cooking spray. Fill large bowl half full with ice cubes; fill with cold water.
BRING 4 quarts of water to a boil in large saucepan; add green beans and salt. Cook, stirring occasionally, for 5 minutes or until crisp-tender. Drain and immediately drop into ice-cold water to stop the cooking. Once cool, drain again.
MELT butter in same saucepan; add mushrooms and garlic. Season with pepper. Cook, stirring frequently, for 5 minutes or until mushrooms are tender. Stir in flour; cook, stirring frequently, for 1 minute. Stir in water; bring to a boil. Reduce heat to medium; add evaporated milk and bouillon. Cook, stirring occasionally, for 10 minutes or until sauce is thickened. Add green beans; stir to coat. Transfer to prepared casserole dish. Sprinkle evenly with French fried onions.
BAKE for 15 minutes or until onions are golden brown and sauce is bubbling around edges. Serve immediately.