Grease the inside of the casserole lightly; this will make it easier to clean later.
Preheat the oven to 400°F (200°C)
Fill a 2 qt. pot 3/4 full with cold water. Bring it to a boil and add 1/2 teaspoon of salt.
Add 1 cup of macaroni. Once it starts boiling, scrape the bottom with a wooden spatula to loosen the
macaroni stuck to the bottom of the pot. Cook until "al dente" (cooked but still firm). Drain and set aside.
In a large skillet melt 2 tbsp. of butter and 2 tbsp. extra virgin olive oil over low heat.
Add 4 tbsp. all purpose flour and 1 tsp. salt. Cook and stir over low heat for 3 to 4 minutes or until
the sauce becomes bubbly and runny.
Remove from heat. When the skillet has cooled down, add 3 cups of milk. Whisk until the flour and milk
is well mixed. Return to the heat. Cook and stir over medium heat until the sauce just starts to boil.
Remove from heat and set aside.
Heat 1 tbsp. olive oil in a skillet over medium heat. Sauté finely chopped onion, celery, and
green bell pepper. Set aside.
Boil diced carrot, broccoli flowerets, and cauliflower flowerets (about 1/2 lb. or 227 g of vegetables
altogether) until cooked. Alternatively, you can use a 1/2 lb. or 227 g of frozen mixed vegetables.
Add cooked macaroni, leftover roast chicken pieces, sautéed vegetables and boiled vegetables to the white sauce
you created earlier. Mix well and pour into the greased casserole. Sprinkle bread crumbs and parmesan cheese
on top of the casserole and bake at 400°F (200°C) for 30 to 40 minutes or until the top is nicely browned.