Preheat oven to 400 F.

Cook macaroni in boiling water with salt according to the package.

Cut chicken into bite-size pieces, and slice onion thinly.

Heat oil in a frying pan at medium heat, and cook chicken until brown. Add onion and a little bit of salt and pepper. Cook further until the onion softens. Set aside.

In a different pot, melt butter at medium heat, add flour, and cook for a couple of minutes until the sauce bubbles. Add milk and whisk well so there are no lumps. When the sauce gets thicken, season with 1/2 tsp salt and pepper.

Set 1/3 of the sauce aside. Add chicken and drained macaroni into the 2/3 of the sauce and mix. Put it in a baking dish, and pour the remaining 1/3 of the sauce over the macaroni. Cover with mozzarella cheese, and bake in the oven on the upper rack for 13-15 minutes.