In a medium saucepan cook macaroni according to package directions, except do not add salt to the water; drain.
Meanwhile, coat a large nonstick skillet with cooking spray; heat skillet over medium-high heat. Add chicken and onion hot skillet. Cook for 5-6 minutes until chicken is no longer pink.
Add semisoft cheese, stir until melted.
In a medium bowl whisk together ilk and flour. Add to chicken mixture. Cook and stir over medium heat until thickened and bubbly. Reduce heat to low. Add cheddar cheese, stir until melted. Add cooked macaroni, cook and stir until heated through (about 1-2 minutes). Stir in spinach. Top with cherry tomatoes.