To make the veal
Soak the dried porcini mushrooms in hot water for 20 minutes, or until soften. Remove the mushrooms, reserve the liquid, and chop the mushrooms finely.
In a large sauté pan over medium heat, heat the olive oil until very hot. Brown the shanks, about 3 minutes on each side, cooking 2 to 3 at a time, depending on the size of the pan. Remove the shanks to a plate.
Drain the grease from the pan and put the heat on low. Add the pancetta and cook until the fat is rendered and the meat starts to brown, about 10 to 12 minutes. Add the carrots, fennel, onions, garlic, red pepper, and chopped rosemary to the pan and cook over low heat for 30 minutes, until the vegetables are soft and any liquid has evaporated. Add the wine, stock, porcini mushrooms, reserved mushroom liquid, and tomatoes and bring to a boil over high heat.
Preheat the oven to 350°. Place the shanks in a large roasting pan and pour the vegetable mixture over them, moving the shanks to let some cooking liquid under them. Braise covered for 2 hours, or until the shanks are very tender.
When the shanks are almost done, in a large sauté pan over medium heat, sauté the porcini, oyster and shitake mushrooms in 2 tablespoons of olive oil for 5 to 6 minutes, or until they have wilted and released their moisture. Remove from pan and let cool.
Remove the cooked shanks to a platter and keep them warm in the oven. Place the braising liquid and the vegetables, in a small saucepan and reduce to about 3 cups. Season if needed, add the mushrooms, and heat through. Set aside.
To make the polenta
Heat the milk and water in a large pot and bring to a boil. While it is simmering, slowly add the polenta. Stir constantly for 6 to 7 minutes, or until the liquid has been absorbed. Remove from heat and stir in the mascarpone.
Serve the polenta on dinner plates, top with the veal shanks, and finish by spooning the mushroom sauce over the shanks.