Place the milk and the clove-studded onion into a small pan over a medium heat. Bring to the boil then switch off the heat, allowing the flavours to infuse into the milk.
In another pan, melt the butter over a medium heat and, when foaming, add the flour. Stir well and cook for four minutes. The mixture will be quite thick to start with. Don't allow it to catch and burn.
Gradually add the onion-infused milk to the butter and flour mixture, stirring constantly, bringing the milk to the boil between each addition of milk. Keep adding the milk slowly until it's all used up.
The finished white sauce is called a bechamel sauce. Simmer the bechamel sauce for five minutes, until it has a rich, smooth consistency and is free from lumps.
Add the grated cheddar to the bechamel and stir again until melted and smooth. Turn off the heat. Once the cheese is added to the bechamel, don't allow it to boil or the cheese will become stringy.
Place the cooked macaroni into a large bowl. Slice the leeks finely, at an angle. (If using large leeks, use only the tender, white part.)
Add the sliced leeks to the bowl with the pasta and season, to taste, with salt and freshly ground black pepper.
Add the cheese sauce to the pasta and leeks and mix thoroughly.
Place the macaroni cheese into a shallow ovenproof dish and sprinkle with the grated parmesan. Put it in the preheated oven to bake until golden brown on top, about 15 minutes.
Serve individual portions straight from the baking dish.