Cook the macaroni
Bring a large pan of salted water to the boil, add the macaroni, give it a stir to separate out the strands, and simmer until just tender, then drain it into a colander.
Make the sauce
Melt the butter in a medium-size non-stick saucepan over a medium heat, add the flour and cook the roux for about 1 minute until it’s floury in appearance and seething nicely. Remove from the heat and gradually work in the milk using a wooden spoon, just a little at a time to begin with. Return the white sauce to the heat, and bring to the boil stirring frequently until it thickens. Add the bay leaf and simmer over a low heat for 10 minutes, stirring occasionally. Remove the bay leaf, add theDijon mustard and give the sauce a whisk to ensure that it is completely smooth and silky, then season to taste with salt, pepper and nutmeg.
Finish the gratin
Towards the end of this time, heat the grill and place a 20 x 30 cm gratin dish or one of equivalent size underneath to warm through. Stir the macaroni into the sauce and gently reheat, then mix in the Gruyere and tip into the hot dish. Scatter over theParmesan, and grill until starting to colour. Serve straightaway.