In a large pot, heat 2 tablespoons of butter and 1 tablespoon of olive oil over medium heat. Sauté pancetta until crispy, then remove and set aside. In a food processor, finely chop onion, carrots, celery, and garlic. In the same pot, brown ground sirloin and Italian sausage over low heat without browning. Mix in the minced vegetables, then add tomato paste, pour in red wine, and scrape up any brown bits. Stir in crushed tomatoes, 1 cup of milk, bay leaves, and cooked pancetta. Simmer for 2.5 to 3 hours, stirring occasionally until the sauce thickens and flavors meld.

Bring a large pot of salted water to a boil and cook lasagna noodles until al dente, about 4 minutes. Drain the noodles well, then gently toss them with a drizzle of olive oil to prevent sticking. Set the noodles aside while you prepare the béchamel sauce.

In a medium saucepan, melt unsalted butter over medium heat. Once melted, whisk in flour and cook for about 2 minutes until it forms a roux. Gradually add warmed whole milk, constantly whisking until the mixture thickens, approximately 5 to 7 minutes. Stir in freshly grated nutmeg and grated Parmesan cheese until melted and smooth.

Preheat your oven to 375°F (190°C). In a greased 9x13 baking dish, spread a layer of béchamel sauce at the bottom. Layer on cooked lasagna noodles, followed by a generous portion of the Bolognese sauce, more béchamel, and a sprinkle of diced mozzarella. Repeat this layering process until all ingredients are used, finishing with a layer of béchamel and a final sprinkle of mozzarella and Parmesan on top.

Cover the assembled lasagna with aluminum foil and place it in the preheated oven. Bake for 1 hour, then remove the foil and continue baking for an additional 15 minutes, or until the top is bubbly and golden brown. Let it rest for 15-20 minutes to set before slicing.