Cook macaroni according to package directions; drain.
Meanwhile, coat a large nonstick skillet with cooking spray; heat skillet over medium-high heat. Add chicken and onion; cook 4 to 6 minutes or until chicken is no longer pink and onion is tender, stirring frequently. (If onion browns too quickly, reduce heat to medium.) Remove from heat. Stir in semisoft cheese until melted.
In a bowl whisk together milk and flour until smooth. Gradually stir milk mixture into chicken mixture. Cook and stir over medium heat until thickened and bubbly. Reduce heat to low. Gradually add cheddar cheese, stirring until melted. Add cooked macaroni; cook and stir 1 to 2 minutes or until heated. Stir in spinach. Top with cherry tomatoes.