Heat olive oil in a large braiser or Dutch oven over medium. Add pancetta; cook, stirring occasionally, until fat is rendered and pancetta is crisp, about 6 minutes. During final 1 minute of cooking time, push pancetta to one side of braiser and add 6 sage leaves to drippings in skillet; cook until leaves become crisp, about 15 seconds per side. Transfer sage leaves to a small plate lined with paper towels; set aside for garnish. Leave pancetta and drippings in braiser. Add mushrooms to braiser and cook, stirring occasionally, until golden, about 4 minutes. Add onion, carrot, and celery; cook, stirring often, until softened, about 5 minutes. Add garlic, bay leaves, rosemary, thyme, and remaining 3 sage leaves; cook, stirring constantly, until fragrant, about 1 minute. Increase heat to medium-high and add ground beef and ground veal. Cook, stirring often to break up meat into small crumbles, until no longer pink, about 5 minutes. Add wine and bring to a boil over medium-high; boil, stirring occasionally, until wine is nearly evaporated, about 15 minutes.
Add stock to mixture in braiser; bring to a boil over medium-high. Cover and reduce heat to low; cook, stirring occasionally, for 1 hour. Uncover and increase heat to medium; simmer, stirring occasionally, until stock is nearly evaporated, about 25 minutes. Stir in cream and cook, stirring constantly, until heated through and well combined, 2 minutes. Remove and discard sage leaves and bay leaves. Stir in salt, pepper, and nutmeg. Remove from heat; cover and set aside.
Bring a large pot of salted water to boil over high. Add pasta and cook according to package directions for al dente. Drain, reserving 1 cup cooking water.
Return sauce in braiser to medium-low; add cooked pasta and cheese. Cook, tossing constantly, until pasta is well coated in sauce and cheese is melted, about 2 minutes, adding reserved cooking water as needed to adjust sauce to desired consistency. Season with additional salt and pepper to taste. Garnish with reserved fried sage leaves. Serve immediately with additional cheese.
