Soak the porcini mushrooms in 1 cup of hot stock until softened, about 30 minutes. Strain the liquid through a coffee filter or a sieve lined with a double layer of cheesecloth. Trim any hard bits from the mushrooms. Set aside the mushrooms and liquid separately.
Season the beef with salt and pepper. In a large casserole, heat 2 tablespoons of the olive oil over medium high heat until hot but not smoking. Brown the beef on all sides, about 15 minutes total. Transfer the beef to a plate. Pour off browning fat. Add pancetta, onions, carrots, and celery, stirring, until they are golden. Add garlic and cook 1 minute more. Season lightly with salt and pepper. Stir in bay leaves, rosemary, parsley, and tomato paste. Add the wine and simmer until it is reduced by half, about 20 minutes.
Add the beef, tomatoes, porcini mushrooms, mushroom liquid, and enough beef stock to come 2/3 up the side of the beef. Bring to a simmer, cover, and cook at a gentle simmer for 2 1/2 to 3 hours, until the meat is tender. Turn the roast every 30 minutes or so.
Transfer the meat to a cutting board and cover with aluminum foil to keep warm. Skim off any fat from the surface of the liquid. Strain the liquid, pressing on the solids with a spoon to extract as much liquid as possible. Discard the solids and return the strained sauce to the pot. Bring to a boil and let cook uncovered for a few minutes to further reduce and thicken the liquid. Season with salt and pepper, to taste. Slice the roast and serve with the sauce and soft polenta.
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