1. Heat a skillet and place the bacon in it. Once they have released their fat, drain them and place them back in the skillet over high heat until they are well grilled. Set aside.
Place a large saucepan over high heat. Melt the 50g of butter in it. Once it is foamy, add the flour all at once and mix vigorously with a spatula until the mixture is smooth and starts to brown slightly. Then gradually pour in the milk, whisking constantly to prevent lumps from forming. Cook over medium heat, stirring constantly, for 5 to 10 minutes, until the béchamel sauce thickens. Season with salt, pepper, and nutmeg. Set aside off the heat.
Preheat your oven to 200 °C (392 °F). Cook the pasta in a large pot of boiling water according to the package instructions, to keep them "al dente". Drain them and pour them into a large gratin dish. Add the bacon, mix well. Pour the béchamel sauce over the pasta, then sprinkle with grated cheese. Bake for about 20 minutes.