Heat olive oil and butter in a large Dutch oven over medium heat.
Cook diced pancetta until crispy, about 5 minutes.
Add onion, celery, and carrot. Cook until soft and fragrant, about 10 minutes.
Add ground beef, pork, and salt. Cook until meat loses raw color but isn't browned.
Pour in milk and simmer until completely evaporated, stirring frequently.
Add a pinch of nutmeg.
Add wine and simmer until evaporated, scraping up bottom bits.
Add crushed tomatoes and bay leaf, stir to combine.
Reduce heat to lowest setting and simmer uncovered for 3 hours, stirring every 30 minutes.
Add small amounts of water if sauce becomes too thick.
Remove bay leaf and adjust salt to taste.
Cook fettuccini in salted water until al dente.
Drain pasta, reserving 1 cup pasta water.
Toss hot pasta with bolognese sauce, adding pasta water if needed.
Serve topped with freshly grated Parmesan.