Heat 1/2 tbsp extra-virgin olive oil in a large pot over medium heat. Add the onion, carrot, and celery and cook for about 5 minutes until the vegetables are soft and fragrant.
Add 1 lb ground beef and cook, breaking it apart with a spoon, until it is fully browned.
Pour in 1/2 cup dry white wine and stir well, scraping up any browned bits from the bottom of the pan. Let it simmer for a few minutes until the liquid has mostly evaporated.
Stir in 1 cup whole tomatoes with juice, sprig rosemary, and bring the sauce to a gentle simmer.
Reduce the heat to low and let the sauce cook slowly for 1 to 2 hours, stirring occasionally, until thick and flavorful.
Stir in 1 cup milk and continue cooking for another 20–30 minutes until the sauce becomes rich and creamy.
Season with salt and black pepper to taste.
Bechamel Sauce
Melt 4 tbsp butter in a saucepan over medium heat until it begins to bubble. Add 4 tbsp flour and stir well to form a smooth paste, cooking it for about 1 minute to remove the raw flour taste.
Gradually pour in about half of the milk while whisking continuously to keep the sauce smooth and lump-free. Once the mixture begins to thicken, add the remaining milk along with 1/2 tsp nutmeg, and a pinch of salt and black pepper.
Keep stirring the sauce over gentle heat until it becomes thick enough to coat the back of a wooden spoon. Remove the saucepan from the heat and set the sauce aside until ready to use.
Assemble the Lasagna
Preheat the oven to 350°F (180°C).
To assemble the lasagna, start by spreading a small layer of bechamel sauce on the bottom of a baking dish. Place a layer of lasagna sheets on top, cutting them if needed so they fit the dish.
Spoon more ragù over the pasta so it is evenly covered, then add about two ladles of béchamel sauce and 2 tbsp of parmesan cheese on top.
Continue layering the pasta, ragù, béchamel and parmesan until all the ingredients are used, making sure to reserve enough béchamel for the final layer. You should end up with about 4–5 layers of pasta.
Spread the remaining béchamel over the top and finish with the parmesan cheese.
Bake the lasagna for about 45 minutes, or until the top is golden and the sauce is bubbling. Let the lasagna rest for 5–10 minutes before slicing and serving.
Notes
Build even layers Try to spread the sauces evenly across each layer. Balanced layers ensure that every bite contains pasta, meat sauce, and creamy béchamel.Finish with cheese on top Adding mozzarella or parmesan on the final layer helps create a golden, bubbly crust that makes the lasagna even more delicious.Let the lasagna rest before slicing After baking, let the lasagna sit for about 10 minutes before cutting into it. This allows the layers to set and makes it much easier to serve neat portions.Lasagna tastes even better the next day Like many slow-cooked dishes, lasagna often tastes even better the following day because the flavors have had more time to develop.