Heat a pan and put in the bacon. As soon as they have returned their fat, drain them and put them back in the pan on high heat until they burn well. Book. Place a large saucepan on high heat. Melt the 50 g butter. As soon as it is sparkling, pour the flour in one go and mix vigorously with a spatula, until the mixture is homogeneous and begins to scorch slightly. Then pour the milk very gradually, while whisking to avoid the formation of lumps. Cook over medium heat, stirring constantly for 5 to 10 minutes while the béchamel thickens, then salt, pepper and add the nutmeg. Reserve off the heat. Preheat your oven to 200 ° C. Cook the pasta in a large volume of water as indicated in the bag, to keep them "al dente". Drain and pour in a large gratin dish. Add the bacon, mix well. Pour the bechamel on all pasta, then sprinkle with grated cheese. Bake for about 20 minutes.