Melt margarine in medium nonstick skillet or saucepan over medium heat. Add mushrooms; cook 4 to 6 minutes or until tender, stirring occasionally. Remove from heat; cover to keep warm.
In medium bowl, combine all omelet ingredients; mix well.
Spray 10 to 12-inch nonstick skillet with sloping sides (omelet pan) with nonstick cooking spray. Heat skillet over medium-high heat until hot. Reduce heat to medium; pour egg mixture into skillet. Cook about 1 minute or until mixture starts to cook around edges. Reduce heat to medium-low; cover and cook 3 minutes. Uncover; lift edges to allow uncooked egg mixture to flow to bottom of skillet. Cover; cook an additional 3 to 7 minutes or until set.
Spoon mushroom filling over half of omelet. Carefully fold other half of omelet over filling. Cut omelet into 3 pieces; slide each piece onto individual plate. Sprinkle with parsley.