1. Cook macaroni according to package directions, omitting any salt; drain.

2. Meanwhile, coat a large nonstick skillet with cooking spray; heat skillet over medium-high heat. Add chicken and onion; cook for 4 to 6 minutes or until chicken is no longer pink and onion is tender, stirring frequently.

3. Remove skillet from heat and stir in semisoft cheese until melted.

4. In a medium bowl, whisk together milk and flour until smooth. Add to chicken mixture; cook and stir over medium heat until thickened and bubbly.

5. Stir in cheddar cheese until melted.

6. Add cooked macaroni and spinach; cook and stir for 1 to 2 minutes or until heated through and spinach begins to wilt.

7. Stir in tomatoes. Serve immediately.