Slice the onion thinly. Wrap spinach in plastic wrap and microwave at 600W for 1.5 minutes, then rinse, drain, and cut into bite-sized pieces. Cut chicken into bite-sized pieces.

Boil water with salt in a pan, add macaroni, cook according to package instructions, and drain.

Wipe the pan, add half the butter, onion, and chicken. Sauté until the onion softens. Remove from the pan and set aside.

In the same pan, add the remaining butter and flour. Stir until the flour loses its raw taste, then gradually add milk while stirring over low heat.

Once the sauce thickens, add diced tomatoes, macaroni, spinach, chicken, and onion. Mix well.