. Combine first 4 ingredients in a large bowl; stir well with a whisk.
. Heat oil in a large nonstick skillet over medium-high heat. Add onion and mushrooms; sauté 9 minutes or until liquid evaporates. Add spinach; cook 4 minutes or until liquid evaporates. Add egg mixture; stir well. Reduce heat to medium-low; cook 8 minutes or until edges are set. Sprinkle with cheese. Wrap handle of pan with foil, and broil 5 minutes or until egg is set and lightly browned.
Note: To speed up the cooking process of a frittata, loosen edges of frittata from pan with a spatula and allow raw egg mixture to run to the bottom of the pan.