HEAT oven to 375°F.
COOK mushrooms and onions in large nonstick skillet sprayed with cooking spray on medium-high heat 5 min. or until crisp-tender. Remove from heat; stir in spinach.
BEAT eggs and milk in medium bowl with whisk until well blended. Stir in dressing. Add to spinach mixture with cheese; mix well. Spoon into pie shell; top with bacon.
BAKE 50 min. or until centre is set and top is golden brown.