HEAT oven to 350°F. Spread 3/4 cup sauce onto bottom of 13x9-inch baking dish; set aside. Mix cheeses, egg and pesto; spoon into large resealable plastic bag. Seal bag. Use scissors to cut off one corner from bottom of bag.
SQUEEZE cheese mixture from bag into both ends of shells; place over sauce in dish. Top with remaining sauce; cover with foil.
BAKE 40 min. or until heated through.