Cook the macaroni in water, adding salt, till just done. Drain, add 1 tsp oil, mix well and keep aside. This ensures the macaroni doesnu2019t stick and clump together after cooling.
Steam cook the chopped vegetables till just done and firm. I microwave them at 100% for about 6 minutes. Keep aside.
In a pan, melt the butter and add the chopped onions. Sautxe9 the onions till soft and translucent. Turn down the heat and add the milk, the cheese slices (torn into pieces) and grated cheese. Stir till the cheese melts completely. Now add the cooked vegetable, herbs, mustard powder, chilli flakes, pepper and salt. Remember that the macaroni and cheese already contain salt. Stir well and add the cooked macaroni. Mix well gently so that the cheese sauce uniformly coats the macaroni. Keep aside.
In a bowl, put the breadcrumbs and the remaining 3 tsps oil and mix well together using your fingers.
Now take a glass dish and grease well. Spread the macaroni mixture in it. Cover with the prepared breadcrumbs to form a layer on top. (This can be made upto this point and then grilled/ baked just before serving.) Now place this under the grill till the top is brown in colour. This takes about 5 -6 minutes in my grill. You can also bake it in the oven. I have never done this because the grill is quick.Serve warm.