PREHEAT oven to 400°F. Heat dressing in large skillet on medium-high heat. Add onions; cook and stir 3 to 5 min. or until tender. Add spinach, corn and peppers; cook and stir 2 to 3 min. or until spinach is wilted. Remove from heat. Add cream cheese, sour cream and Parmesan cheese; mix well.
SPOON into 9-inch pie plate.
BAKE 20 min. or until heated through. Serve with the crackers.