HEAT oven to 350ºF.
COOK pasta shells as directed on package, omitting salt.
MEANWHILE, brown beef with onions in large skillet; drain. Remove from heat. Mix cream cheese spread, 2 Tbsp. milk and pesto until blended; stir 3/4 cup into meat. Add ricotta, 1/2 cup mozzarella and Parmesan; mix well.
DRAIN shells; stuff with meat mixture. Spread 1 cup sauce onto bottom of 13x9-inch baking dish; top with shells and remaining sauce. Cover.
BAKE 25 min. Mix remaining cream cheese mixture and milk; drizzle over shells. Top with remaining mozzarella; bake, uncovered, 5 min. or until melted.