HEAT oven to 400ºF.
COOK potatoes in boiling water in large saucepan 15 min. or until tender.
MEANWHILE, brown meat with onions in large skillet on medium-high heat; drain. Add tomatoes, tomato paste, Italian dressing and Worcestershire sauce; mix well. Simmer on medium-low heat 5 min. or until slightly thickened, stirring occasionally. Spoon into 8-inch square dish sprayed with cooking spray.
DRAIN potatoes; return to pan. Mash lightly. Add cream cheese; stir until melted. Stir in ranch dressing; spoon over meat mixture.
BAKE 20 min. or until casserole is heated through and top is golden brown.