Preheat the oven to 300°F.
Using a clean coffee grinder or spice mill, grind the porcini mushrooms into a powder.
Season the beef liberally with salt and pepper. Heat a Dutch oven or other wide ovenproof pot over high heat with the olive oil then add the beef and cook, turning the meat occasionally, for about 10 minutes, or until well-browned on all sides. Add the wine and stir to scrape up any browned bits on the bottom of the pot. Add the broth, mushrooms, shallots, garlic, thyme and porcini powder, and bring to a simmer.
Cover the pot and place it in the oven. Braise the beef, turning the pieces of meat over halfway through the cooking, for 3 hours.
Uncover the pot and continue cooking the beef for about 1 hour, or until it is tender enough to pull apart with a spoon and the liquid has reduced by about one-third. Discard the thyme stems.
For the Horseradish Mascarpone, in a small bowl, mix the mascarpone, horseradish, heavy cream and lemon juice until blended. Season to taste with salt and pepper.
To serve, transfer the beef to four shallow serving bowls, spoon the mushrooms and braising liquid over and serve topped with a dollop of the horseradish mascarpone.