In a medium saucepan cook macaroni according to package directions, except do not add any salt to the water; drain.
Meanwhile, coat an unheated large nonstick skillet with cooking spray. Preheat skillet over medium-high heat. Add chicken and onion to skillet. Cook for 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 skillet from heat. Stir in semisoft cheese until melted.
In a medium bowl whisk together milk and flour until smooth. Add all at once to chicken mixture. Cook and stir over medium heat until thickened and bubbly. Reduce heat to low. Stir in cheddar cheese until melted. Add cooked macaroni; cook and stir for 1 to 2 minutes or until heated through. Stir in spinach and top with cherry tomatoes. Serve immediately.