Heat olive oil in a Dutch oven over medium heat and cook pancetta until fat is rendered and meat is crisp.
Add onion, carrot, and celery; cook until translucent and soft, approximately 10 minutes.
Increase heat and add ground beef and pork; sear until browned, breaking meat into small crumbles.
Stir in tomato paste and cook for 3 minutes to deepen flavor.
Pour in red wine and deglaze the pan, simmering until the liquid has reduced by half.
Add crushed tomatoes and 250ml of milk; reduce heat to low and simmer for at least 2 hours.
Prepare the béchamel by melting butter in a heavy saucepan, whisking in flour for 2 minutes without browning.
Gradually whisk in remaining 750ml milk; cook over medium-low heat until the sauce coats the back of a spoon.
Season béchamel with nutmeg, salt, and pepper.
Preheat oven to 190 degrees Celsius.
Spread a layer of ragù in a 9x13 inch baking dish, then cover with pasta sheets.
Apply a layer of ragù, a layer of béchamel, and a dusting of Parmigiano-Reggiano; repeat for 4 to 5 layers.
Finish with a final layer of béchamel and a generous coating of Parmigiano-Reggiano.
Bake for 35-40 minutes until golden brown and bubbling; rest for 20 minutes before service.