For the lasagna, finely dice the onion and sauté in a little oil. Add minced meat and fry until hot.
Season with salt and pepper and deglaze with tomato sauce. Remove the pan from the heat and season the Bolognese sauce with oregano.
For the béchamel sauce, melt butter over low heat. Stir flour into melted butter and add a sip of milk and stir.
Slowly stir in all the milk, stirring constantly. Bring mixture to a boil and then remove from heat.
Add Parmesan cheese, salt and nutmeg to taste. In an ovenproof dish, layer first the tomato sauce, then the béchamel sauce and then the lasagna leaves.
Finish with the béchamel sauce and sprinkle with Emmentaler. Bake at 160 °C for about 30 minutes. Then increase the temperature to 220 °C and bake for another 10 minutes until the cheese is golden/brown.