This rich, complex sauce smells and tastes like fall—with its earthy mushrooms, smoky pancetta, and sweet woodsy rosemary. Serve it over pasta, ideally pappardelle, and freeze any extra (you'll have a lot of sauce). This recipe uses a smaller quantity of milk than is traditional, since milk can curdle in the slow cooker. Be sure to use high-quality pancetta and feel free to vary the kind of mushroom.
Tip:To adapt a slow-cooker recipe to a conventional oven, follow these guidelines: add more liquid, to accommodate for greater evaporation; bring the dish to a boil over high heat in a Dutch oven, then cover the pot and put in a 350°F oven. Plan on the dish taking roughly half the time to cook in the oven as it would in the slow cooker.
Add the dried mushrooms to a small bowl and cover with very hot water; let sit for 15 minutes, then drain and finely chop. In a large bowl, mash the bread and milk together until a smooth paste forms. Gently knead in the beef, salt, and pepper, and mix well.
Add the butter to a medium-size, heavy saucepan and heat over medium-high heat. When melted, add the pancetta and sauté until browned, about 8 minutes. Pour off all but about 2 Tbs. of the fat. Add the meat mixture, and cook until browned, about 10 minutes. Add the fresh porcini and cook until softened, about 2 minutes. Stir in the dried mushrooms, carrots, celery, onions, garlic, and nutmeg. Sauté until the vegetables are slightly softened and aromatic, about 3 minutes. Add the tomato paste and flour, and cook until the flour is no longer visible, no more than 1 minute. With the pan off the heat, carefully add the wine. Then return the pan to high heat and stir well, scraping the bottom of the pan with a wooden spoon to release any food bits. Cook for 2 minutes, then stir in the tomatoes and stock, and cook for another minute. Pour into the slow cooker.
Cover the slow cooker and cook on low until the meat has cooked through and the sauce is aromatic and very flavorful, about 6 hours. Spoon the fat off the surface, stir in the rosemary, break up the meat more with a fork, and serve with pasta.