Heat 10-inch nonstick skillet with flared sides (omelet pan) over medium-high heat. Reduce heat to medium; spread spinach in skillet. Pour egg mixture over spinach. Cook about 1 minute, lifting edges occasionally to allow uncooked egg mixture to flow to bottom of skillet, until mixture begins to cook around edges. Reduce heat to medium-low; cover and cook 3 to 6 minutes or until set.