Heat half the butter in a frying pan over a medium-low heat and gently cook the leeks for about 10 minutes, until soft. Add the speck or bacon, turn up the heat and fry until crisp. Cook the macaroni in plenty of salted boiling water until al dente, according to the instructions on the packet, and drain. Melt the remaining butter in a medium saucepan and stir in the flour. Cook for a couple of minutes, then whisk in the milk and cook, stirring, until the sauce has thickened. Stir in 200g of the cheese, the mustard and a grating of nutmeg. Taste and season if necessary. Add the pasta, leeks and bacon and toss together well. Heat the grill to medium. Butter a baking dish and tip the pasta in. Mix the breadcrumbs with the rest of the cheese, sprinkle on top of the pasta, and grill until golden and bubbling.