To prepare filling, heat oil in a medium nonstick skillet over medium heat. Add onion; sauté 5 minutes. Add garlic; sauté 30 seconds. Combine onion mixture, chicken, and next 7 ingredients (chicken through spinach) in a medium bowl.
To prepare sauce, combine Chicken Stock and the next 4 ingredients (stock through ground red pepper) in a medium saucepan; bring to a boil. Lightly spoon flour into a dry measuring cup; level with a knife. Place flour in a small bowl; gradually add milk, stirring with a whisk until blended. Add to pan; bring to a boil. Cook 2 minutes or until thick, stirring constantly with a whisk. Stir 1 cup sauce into chicken mixture in bowl. Add 1/2 cup cheese to sauce in pan, stirring well until mixture is smooth.
Spoon chicken mixture into cooked manicotti (about 1/3 cup per manicotti). Arrange stuffed manicotti in a 13 x 9-inch baking dish coated with cooking spray. Pour cheese sauce over manicotti. Bake at 375° for 45 minutes or until browned and bubbly.