Preheat the oven to 375. Place mushrooms, broccoli and onions in skillet sprayed with cooking spray; cook on medium heat 5 minutes or until vegetables are tender, stirring occasionally. Add spinach. Remove from heat; set aside.
Beat eggs, dressing and milk in medium bowl with wire whisk until well blended. Stir in vegetable mixture and cheese; pour into pie shell. Place pie shell on baking sheet.
Bake on backing sheet 40-45 minutes or until center of quiche is set and top is golden brown. Let stand 10 minutes before cutting into slices to serve.