Cook pasta as directed on package, omitting salt.
Meanwhile, melt butter in Dutch oven or large deep skillet on medium heat. Add onions; cook and stir 4 min. or until crisp-tender. Add frozen vegetables; cook 4 min. or until thawed, stirring frequently. Stir in flour until blended; cook and stir 1 min. Gradually stir in milk; bring to boil. Cook and stir 2 min. or until thickened. Remove from heat. Stir in turkey, soup and sour cream.
Drain pasta. Add to sauce; stir until evenly coated. Spoon into 2 (13x9-inch) pans sprayed with cooking spray; top with cheese. Cover.
Bake 30 min. or until heated through, uncovering for the last 10 min.