Either use ready-made lasagne sheets or make them at home as follows: Mix the flour, eggs and salt and then pull the dough, or roll out the dough with a rolling pin until it is very thin. Cut the dough into large squares and boil them in salted water for 1-2 minutes.
For the ragù:
Heat a little olive oil in a sauce pan and cook and stir the onion, carrot and celery until the onion is translucent, about 5 minutes. Stir in the minced beef, season with salt and pepper and stir in the chopped tomatoes. Bring the sauce to the simmer and cook the sauce until thickened, 1 to 2 hours, depending on how much liquid comes from the tomatoes. When the sauce has finished cooking, add a sprinkling of oregano.
For the Bechamel sauce:
Bring the milk to the boil in a saucepan. Melt the butter in another saucepan and whisk in the flour, nutmeg and salt. Cook until the flour becomes a bit golden. Start pouring in the boiled milk a little at a time, whisking constantly over low heat until you get a dense sauce.
Preparing the lasagne:
Preheat the oven to 180 C / Gas 4. Grease a baking dish.
Spread a bit of the ragù on the bottom, add a layer of lasagne sheets and a layer of Bechamel sauce. Add another layer of sheets, a layer of ragù, a layer of mozzarella and a layer of Bechamel sauce. Continue like this until you have used up all the ingredients. The last layer should be the ragù.
Bake in the preheated oven for around 40 minutes. Take out the lasagne and add some more mozzarella on top and return to the oven until the top is golden brown, about 15 minutes.