Make the Bolognese Sauce:
Heat olive oil in a large pot over medium heat. Add onion, carrots, and celery, and cook until softened, about 5 minutes.
Add garlic, ground beef, and ground pork. Cook until browned, breaking up the meat with a wooden spoon.
Stir in milk and simmer until absorbed. Add wine and simmer until reduced.
Add crushed tomatoes, tomato paste, salt, pepper, and nutmeg. Simmer on low heat for 1-2 hours, stirring occasionally.
Prepare the Béchamel Sauce:
In a medium saucepan, melt butter over medium heat. Stir in flour and cook for 1-2 minutes.
Gradually whisk in warm milk, ensuring no lumps form. Cook until thickened, about 5-7 minutes. Season with salt, pepper, and nutmeg.
Assemble the Lasagna:
Preheat the oven to 375°F (190°C). Spread a thin layer of Bolognese sauce at the bottom of the baking dish.
Add a layer of lasagna sheets, followed by Bolognese sauce, béchamel, mozzarella, and Parmesan. Repeat layers, finishing with béchamel and cheese on top.
Bake the Lasagna:
Cover the dish with foil and bake for 25 minutes. Remove the foil and bake for another 20 minutes, or until bubbly and golden.
Let the lasagna rest for 10-15 minutes before slicing and serving.