Mac & cheese aux trois oignons

Start by caramelizing the onions; that is melt half of the butter on a skillet and when it will be bubbling add the sliced onions, sprinkle with a little salt and cover. Let the onions cook over low flame stirring every now and then adding some water if the onions start to burn or look dry. At the end the mixture should be a nice caramel brown color and be moist but not watery.
While the onions are cooking you can start preparing the béchamel sauce. In a heavy-bottomed saucepan prepare the roux by melting the butter with the flour. Stir the paste with a whisk until it will start to have a nutty smell but it will still be blond in color. Whisking vigorously to avoid lumps, add the milk and, if using, the bay leave with the allspice. Let simmer slowly till the mixture will be reduced of 1/3 (you should have a scant cup of finished sauce), this should take roughly 20-30min.
When the béchamel sauce will be almost ready, put the water for the pasta to boil and then cook the pasta (do not forget to salt the water).
While the pasta is cooking, remove the spices from the sauce (if used) and add the thinly chopped cheese whisking till it will be all melted (if the sauce has cooled down too much, warm it up slightly on a low flame while stirring). If the sauce should get stringy (that should not happen if the cheese is quite old), add 1-2 drops of lemon juice and stir. When the cheese sauce will be ready, add the caramelized onions and season with salt, freshly ground black pepper and grated nutmeg.
In a little saucepan lightly toast the nigella seeds (they will start fuming when they will be ready).
When the pasta will be ready, season it with the cheese sauce and the white part of the onions. Dish the pasta and sprinkle it with the nigella seeds and the julienned onions greens before enjoying your mac&cheese.