Cook mostaccioli to desired doneness as directed on package. Drain; cover to keep warm.
Meanwhile, in medium bowl, combine yogurt and cornstarch; mix well. Set aside.
Heat oil in large skillet or Dutch oven over medium-high heat until hot. Add beef and onion; cook and stir until beef is browned. Add flour; stir until well blended. Add all remaining ingredients except mostaccioli and yogurt mixture. Cover; cook over medium heat for 5 to 8 minutes or until vegetables are crisp-tender, stirring occasionally.
Remove skillet from heat. Gradually stir 2 cups beef mixture into yogurt mixture, stirring constantly. Return beef and yogurt mixture to skillet; cook and stir over medium heat until bubbly. Add cooked mostaccioli to beef mixture; toss to combine.