Heat oven to 350°F. Spray 12x8-inch (2-quart) glass baking dish with nonstick cooking spray. Place potatoes, mushrooms and half of the french fried onions in sprayed baking dish.
In large bowl, combine eggs, milk, green onions, salt, pepper and cheese; mix well. Pour over potato mixture in baking dish. Spray sheet of foil with nonstick cooking spray; cover dish tightly with foil, sprayed side down.
Bake at 350°F. for 35 to 40 minutes or until knife inserted in center comes out clean. Uncover; sprinkle with remaining french fried onions. Bake, uncovered, an additional 2 to 3 minutes. Let stand 10 minutes before serving.