1Heat oven to 400°F. Spray 13x9-inch pan with cooking spray. In 10-inch skillet, cook mushrooms, onion and garlic in butter over medium heat about 5 minutes or until onion is tender; drain.
2In pan, mix mushroom mixture, cheese, walnuts and spinach; spread evenly. In medium bowl, stir all remaining ingredients with wire whisk or fork until blended. Pour into pan.
3Bake 25 to 30 minutes or until knife inserted in center comes out clean. Cool 30 minutes. To serve, cut into 9 rows by 3 rows.