Preheat oven to 350 degrees. Spray a large skillet with cooking spray and sauté the onions, garlic and mushrooms in ½ cup of water until water is completely evaporated. Remove from pan and place in a medium bowl and let cool. When cooled, add the thawed green beans to the onion mixture. In a small bowl, mix all the ingredients for the sauce. When well-blended, add to the bean mixture. Place in a baking dish and cover with the topping. Bake uncovered for 35 minutes.