Cook broccoli and mushrooms in skillet sprayed with cooking spray on medium heat 8 to 10 min. or until broccoli is crisp-tender, stirring occasionally.
Remove from heat.
Line 9-inch pie plate with pie crust; fold over and crimp edge.
Whisk eggs, dressing and milk in medium bowl until blended. Stir in cooked vegetables, VELVEETA and onions; pour into crust.
Bake 40 min. or until center is set and top is golden brown.