Heat oven to 375ºF. Place meatballs on aluminum foil-lined baking sheet. Bake 15 minutes; gently turn meatballs over and bake an additional 13 to 17 minutes until internal temperature reaches at least 165ºF.
Place onion, celery, carrot, garlic, parsley, oregano and red pepper in bowl of food processor or blender. Pulse until finely minced. Sauté vegetables in olive oil over medium-high heat for 5 minutes.
Stir tomato sauce, chicken stock and tomato paste into vegetables. Salt to taste. Simmer sauce 20 minutes, stirring occasionally.
Meanwhile, cook pasta according to package directions; drain. Toss cooked meatballs with 3/4 cup sauce.
Toss pasta with remaining sauce; place in 11x7x2-inch baking pan. Gently stir in 3/4 cup mozzarella. Arrange meatballs on top of ziti; sprinkle with remaining 1/2 cup cheese.
Bake at 375ºF, uncovered, for 20 to 25 minutes until cheese is melted and bubbly. Let stand 10 minutes before serving.
Serve meatballs with side of Ziti sprinkle with remaining ½ cup cheese.