Sauté the onion and garlic in a little oil over medium heat for about 5 minutes until soft and translucent.
Add the minced beef and brown well. Stir in tomato purée and cook for 2 minutes. Add canned tomatoes and oregano, season with salt and pepper. Simmer the meat sauce for at least 30 minutes.
Make the béchamel: melt the butter, stir in flour and cook for 2 minutes. Gradually add milk, whisking constantly, until you have a smooth, thick sauce. Season with salt, pepper and freshly grated nutmeg.
Preheat the oven to 180 °C (fan 160 °C).
Spread a little béchamel in the base of an ovenproof dish. Layer lasagne sheets, meat sauce, béchamel and a little Parmesan. Repeat for 3–4 layers, finishing with béchamel and a generous layer of Parmesan.
Bake for 45–50 minutes until golden and bubbling. Leave to rest for 10 minutes before serving.