Pre-heat the oven on 220 degrees. Start by putting a pot with abundant water to boil with coarse salt for the pasta. When it boils cook the pasta but drain it a couple of minutes before the instructions on the packet. This is because we will continue cooking the pasta in the oven and if it's fully cooked already it will get overcooked.
In a non stick pot start by melting the butter for the bechamel on low heat. When the butter is melted add the flour (half at the time) and keep on mixing it so that it makes an elastic mix. You should use a collander in the original recipe when adding the flour to avoid lumps although I almost always skip this step. This base is called roux (French term for equal part of fat and flour and is used as the basis for a lot of thickening tricks in cooking).
You will now add the milk little at the time and incorporate it into the roux. Now the proper way to do this is to warm the milk (you can do this in the microwave) so that the bechamel is less likely to get lumpy. To be honest I don't always bother with this step but if you are attempting it for the first time maybe stick to it.
Keep on adding the milk a bit at the time until you use if all up - don't worry if the mixture looks too liquid to start with leaving it on the fire will make it thicken. When you are done with the milk add salt and nutmeg. Try the flavour as it often takes more salt and nutmeg than you think. Leave on the fire to cook for approximately 5-6 minutes on low heat.
Now leaving the bechamel on the side means that it gets a patina on top if you turn the heating off so if you want to make it in advance and leave it aside a baker's trick is to stuck cling film to the surface of the sauce.
Aside cook the vegetables in a pan with oil. Cook the onions first with a pinch of salt on low heat. The salt will make the onion "sweat" releasing the water without burning them and cooking them slowly. Add the garlic thinly chopped.
Add the diced vegetables and cook on medium heat for 5-7 minutes until cooked but still slightly crunchy.
Add the vegetables to the drain pasta, put it in an oven proof tray and mix it well. Pour the bechamel evenly on top of the pasta so that it's completely covered and top with the grated cheese. Cook in the oven on the top shelf for 5-10 minutes until the cheese melted and serve hot.
