Slow-Cooker Bolognese Sauce with Pancetta, Porcini, and Rosemary

3 slices white bread

1 cup whole milk

Scant 3 lb. ground beef chuck or meatloaf mix, at room temperature

2 tsp. kosher salt

15 grinds black pepper

2 Tbs. unsalted butter

1/2 lb. pancetta, coarsely chopped

1 cup finely chopped porcini mushrooms (about 5 large)

2 cups finely chopped carrots (about 4 medium)

1 cup finely chopped celery (about 4 stalks)

1 cup finely chopped red onions (about 1 small)

1 Tbs. finely chopped garlic

1 tsp. ground nutmeg

1/2 cup tomato paste (about 5 oz.)

1/4 cup all-purpose flour

1 cup dry red wine, such as Cabernet Sauvignon

3/4 cup canned diced tomatoes, with juice

1/2 cup low-sodium chicken stock

2 Tbs. minced fresh rosemary leaves

1 lb. pappardelle pasta, cooked according to the package directions